bilby version information is out of date if `--tags` are not pulled

@michael.williams and @colm.talbot, there is a slightly confusing behaviour with the new version tool.

I just pulled the master branch at b6fa00a8 and pip installed. After this, I get

Successfully installed bilby-1.1.6.dev157+gb6fa00a8

And the bilby.__version__ attribute also claims v 1.1.6.

This is because a simple git pull does not pull down tags. You need to run git fetch --all --tags and then install. I think we need to add this to the documentation somewhere as people will likely get confused.