Run checkdoc automatically for PRs

[bbatsov]

We should invoke checkdoc on PRs to avoid the need for tickets like #1071.

Setting this up in travis should not be too complex.

[EricGebhart]

Do you really want to just turn it on? If cider-interaction.el is any indication there are going to be a lot of errors everywhere. There were 50 in cider-interaction, I cleaned up dozens of them and ended up with 46. I suppose it will at least make everyone look and not introduce new errors.

[bbatsov]

Obviously we'll have to fix #1070 first. When the project was starting out there was a time when the entire codebase was checkdoc-compliant. There was also a Travis check for the docstrings. I had to turn it off for a reason I don't recall right now and afterwards I forgot to restore it.

[bbatsov]

Reference to the old check - https://github.com/clojure-emacs/cider/pull/262/files

Probably I removed it when I migrated the project to cask.

[bbatsov]

Duplicate of #1352.