Build against go1.13

This MR adds support for go 1.13 to gitlab-shell.

The TLS implementation in go1.13 has started rejecting 512-bit RSA keys, which I'm a big fan of. However, we did use some in our tests, so they needed replacing. I used a 2048-bit key with 10-year validity:

Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number:
            1d:53:53:9b:2c:f7:a7:ee:f1:48:43:24:49:f8:4f:cf:80:59:7e:ac
        Signature Algorithm: sha256WithRSAEncryption
        Issuer: C = US, ST = California, L = The Cloud, O = My Company CA
        Validity
            Not Before: Sep 20 10:47:59 2019 GMT
            Not After : Sep 17 10:47:59 2029 GMT
        Subject: C = US, ST = California, L = The Cloud, O = Demo, CN = My Certificate
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
                RSA Public-Key: (2048 bit)
                Modulus:
                    00:9a:d7:b7:1b:f7:83:fa:ab:9a:9b:0c:85:97:ed:
                    a3:12:8f:27:81:20:13:d0:55:9e:63:96:40:c1:1b:
                    dd:17:81:60:b5:30:4b:bd:b0:1d:4e:7c:a2:ee:91:
                    cc:7a:c0:92:52:4c:b1:5c:71:cf:6b:33:38:48:35:
                    7a:ba:2b:b9:2c:d2:85:d2:2c:ff:35:8e:fa:ac:bb:
                    45:a2:a4:86:52:0c:a0:4d:91:d5:6d:9e:8d:45:70:
                    8d:51:9d:0f:b2:50:fd:e7:03:82:5a:f4:bd:b7:dc:
                    60:35:7a:f2:eb:a9:3c:fa:67:d9:36:11:3e:70:54:
                    1f:ac:ef:e9:37:95:a9:36:e1:b2:59:37:ca:95:44:
                    30:e4:d5:4b:de:6a:1b:8f:7b:52:8c:8f:b0:cd:2a:
                    5b:3c:cb:db:4d:0a:01:89:9f:d5:1e:4c:b2:73:5e:
                    58:76:b6:de:3f:01:41:d9:d2:5b:c4:4f:d7:82:cc:
                    a4:fd:3c:16:49:41:67:02:ce:a2:d7:1d:ad:fb:8d:
                    b7:34:89:bd:ac:80:d3:75:6d:98:60:92:57:bf:73:
                    14:71:d0:c8:c4:99:d8:d1:b7:86:78:f3:08:ca:69:
                    fd:64:84:54:26:d2:bf:65:79:b0:31:be:76:bf:bd:
                    3e:61:37:2c:1b:ae:ee:4a:6d:f5:09:1f:23:36:df:
                    2a:a5
                Exponent: 65537 (0x10001)
        X509v3 extensions:
            X509v3 Basic Constraints: 
                CA:FALSE
            X509v3 Subject Key Identifier: 
                F1:67:D4:8E:46:62:30:0E:CF:74:4D:6F:14:5C:85:61:0E:CB:78:92
            X509v3 Key Usage: 
                Digital Signature, Key Encipherment
            X509v3 Extended Key Usage: 
                TLS Web Client Authentication, TLS Web Server Authentication
            X509v3 Subject Alternative Name: 
                IP Address:127.0.0.1, DNS:localhost
    Signature Algorithm: sha256WithRSAEncryption
         7f:82:2a:f7:84:ae:f5:39:64:45:e3:12:e3:1d:8d:e7:16:6b:
         bb:d6:28:29:0b:6b:af:1a:96:49:16:e9:a2:1e:4b:c2:d9:3d:
         ac:93:f2:ea:ae:08:10:f5:f3:0e:c4:48:d5:85:82:65:ed:45:
         c3:bc:dd:88:d7:e6:a0:54:71:8e:ce:7d:30:12:7d:61:5a:dc:
         15:ec:9e:00:82:e0:2e:20:22:45:c1:ad:87:64:c7:a6:29:02:
         9c:bc:66:9e:21:87:51:45:f5:59:f8:e6:70:31:18:30:8b:3a:
         05:9f:7c:37:ae:15:5b:0b:6e:66:a7:09:9c:93:e6:42:ce:b8:
         94:3a:e0:5b:da:33:eb:ff:21:3c:c7:96:e7:ca:24:f0:f8:e4:
         88:6f:84:fa:ca:c1:69:44:68:2b:03:5e:ee:f7:cf:45:5c:aa:
         e3:93:7a:ef:f8:ee:1f:ee:28:e9:00:5d:58:fe:80:0a:30:cc:
         dd:19:f1:c9:6c:96:d1:2f:ac:97:78:b7:10:01:a0:b7:03:3a:
         44:80:e1:38:6d:b7:f0:73:88:88:0f:c8:ff:e0:82:98:84:ec:
         be:e8:f3:ea:33:0d:6c:b7:68:3d:f9:da:b5:9e:df:e6:6f:43:
         ba:83:ac:1a:56:00:65:b2:df:3a:5d:8d:cb:08:0e:d2:30:25:
         9c:72:46:11
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----

Additionally, the TLS failures so created exposed some ordering issues in the gitlabnet client tests. They were panicking in an attempt to access the response.Body while response was nil. So I reordered those at the same time.