Override the way gems are installed on Travis

It seems that Travis has made a change where instead of installing gems
globally they are installed to `vendor/gems`. This happens because
all of our Appraisal Gemfiles have associated lockfiles that are present
in the repo and Travis picks up on this and choose to run `bundle
with `--deployment`. This breaks our acceptance tests as they assume
that gems are installed globally.

To get around this we need to define our own `install` command.

(cherry picked from commit 309d1ff5aa)

.travis.yml

@@ -2,6 +2,7 @@ language: ruby
 sudo: false
 cache: bundler
 script: "bundle exec rake"
+install: "bundle install --jobs=3 --retry=3"
 
 env:
   - DATABASE_ADAPTER=sqlite3