Fix failing Travis CI setup

The builds are failing due to a strange error where rake cannot find
other dependencies.

    bundler: failed to load command: rake
    LoadError: cannot load such file -- bundler/dep_proxy

It appears this is a known issue (travis-ci/travis-ci#9062) which links
to a Ruby Gem issue (rubygems/rubygems#2055).

This implements a hopefully simple fix which ensures bundler is updated
on the system, and that the installed version of Rails has the bundler
gem available.

Author: cupakromer

.travis.yml

@@ -2,6 +2,9 @@ language: ruby
 bundler_args: --binstubs --jobs=3 --retry=3
 cache: bundler
 sudo: false
+before_install:
+  - gem update --system
+  - gem install bundler
 script: bin/rake
 rvm:
   - 2.5.0