Fix travis build
Fix travis build

Now Travis uses bundler 2.0+ and it causes some build fails since we
still support Rails 4.

This commit set Travis to not use a bundler version higher than 2.0.

More info: https://docs.travis-ci.com/user/languages/ruby/#bundler-20

rubyrider/devise,rubyrider/devise,Qwiklabs/devise,plataformatec/devise,plataformatec/devise,rubyrider/devise,Qwiklabs/devise