Run `gem update --system` before install on CI
Run `gem update --system` before install on CI

There's an issue with the combination of Bundler `1.16.1` and rubygems `2.7.3`.
This command will install rubygems `2.7.4` in order to fix it.

https://github.com/travis-ci/travis-ci/issues/8978

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