Move development reqirements to dev-requirements.txt

This is backport of !637 to bst-1.2.

There were a couple of differences between the requirements listed in
the `master` branch and the `bst-1.2` branch so I have left them
untouched.

For reference, here are the differences between them:

```diff
--- a/dev-requirements.txt
+++ b/dev-requirements.txt
@@ -2,8 +2,9 @@
 # random crashes with 4.4.2
 coverage == 4.4.0
 pep8
+pylint >= 1.8 , < 2
 pytest >= 3.1.0
-pytest-cov >= 2.5.0
+pytest-cov
 pytest-datafiles
 pytest-env
 pytest-pep8
```