panic when config.toml is empty
% git log -1
a806da2 Merge branch '111-watchkey-fix' into 'master'
% echo > config.toml
% ./gitlab-workhorse -config config.toml
2017/03/13 10:42:32 Starting gitlab-workhorse v1.4.0-10-ga806da2-20170313.094110
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0xb8 pc=0x13f0f46]
goroutine 1 [running]:
gitlab.com/gitlab-org/gitlab-workhorse/internal/redis.dialOptionsBuilder(0x0, 0x19011e0, 0x0, 0x0)
/foobar/gitlab-development-kit/gitlab-workhorse/src/gitlab.com/gitlab-org/gitlab-workhorse/_build/src/gitlab.com/gitlab-org/gitlab-workhorse/internal/redis/redis.go:72 +0x26
gitlab.com/gitlab-org/gitlab-workhorse/internal/redis.DefaultDialFunc(0x0, 0xb)
/foobar/gitlab-development-kit/gitlab-workhorse/src/gitlab.com/gitlab-org/gitlab-workhorse/_build/src/gitlab.com/gitlab-org/gitlab-workhorse/internal/redis/redis.go:84 +0x2f
main.main()
/foobar/gitlab-development-kit/gitlab-workhorse/src/gitlab.com/gitlab-org/gitlab-workhorse/_build/src/gitlab.com/gitlab-org/gitlab-workhorse/main.go:136 +0x6e5