Some other minor changes:

* This doesn't actually build with GHC 7.4/`base-4.5` due to the use
  of `LambdaCase`, so raise the lower version bounds on `base`
  accordingly.
* The `Data.Monoid` import is completely redundant, even on older
  GHCs. This patch removes it to avoid warnings.

Rework linters and fix submodule linter

See merge request !2

Previously these expected their first argument to be the path to a .git
directory since they were run from bare repositories (on the gitolite
server). However, these are no longer run on the upstream bare
repositories but instead are run on full working directories.
Consequently the linters now take the path to the working directory
(although a .git directory will also work).

We also adjust the submodule linter to look at the branches of the
working directory instead of the bare gitolite reposotories.

Add CI

See merge request !1

These scripts are now in the GHC repository (IIRC `.gitlab/linters`)

...and thus breaking the EOL-validation

This checks the commit msg style convention, and is inspired by

  http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html