Add pre-push hook to lint .travis.yml file.

[guidomb]

If there is a .travis.yml file and the travis gem is available with you lint the .travis.yml before pushing.

Also after bundler install all the required gems we you check if the travis gem was installed. If not install it. If there is no Gemfile or bundler is not available we you tell the user to install the travis command line if there is a .travis.yml file.

[guidomb]

The problem with this is that travis lint currently does not differentiate between errors and warnings and in the case of iOS the suggested .travis.yml file has warnings.