Review PR in travis with nox-review

This PR enable the use of travis for pull requests.

For each pull requests, each package that is modified by the pull requests (has a different path) is rebuild to check nothing is broken.

One example of the result can be found here: https://github.com/madjar/nixpkgs/pull/1, https://travis-ci.org/madjar/nixpkgs/builds/35873590.

Also, for each commit, the full expression is evaluated, in order to provide fast feedback on broken evaluation (because broken evaluation lead to wrongly failed PR).

This PR should only be merged by a repo admin, who should then activate travis for nixpkgs.