Update the bundler gem during Travis before_install
Update the bundler gem during Travis before_install

This should avoid the error with Bundler 1.13.2:
/Users/travis/.rvm/gems/ruby-2.0.0-p648/gems/bundler-1.13.2/lib/bundler/rubygems_ext.rb:45:in `full_gem_path': uninitialized constant Bundler::Plugin::API::Source (NameError)

mattrubin/SwiftGit2,SwiftGit2/SwiftGit2,mattrubin/SwiftGit2,SwiftGit2/SwiftGit2