Bug fix ismergeable

Hi, 

I have noticed that you are using `merge_commit_sha` to decide if a pull request is mergeable or not. According to github API this is going to be deprecated[1] and they suggest that someone should use `mergeable`  attribute[2]. So I have re-factored at controllers.js the logic that determines if a pull request is mergeable or not. 

Moreover, I thought of re-factoring a little your ProxyView class at app.py.


[1]https://developer.github.com/changes/2013-04-25-deprecating-merge-commit-sha/
[2] https://developer.github.com/v3/pulls/#mergability