Enable Gitlab Syntax Highlighting

The highlighting library used by Gitlab can actually highlight Coq code correctly [1]. We just need to let it know that *.v files are Coq files [2].

References:
[1] http://rouge.jneen.net
[2] https://docs.gitlab.com/ee/user/project/highlighting.html

CC: @sbozhko @mlesourd @sophie

merged