Tell Travis CI to execute rspec with bundle

After adding the Gemfile.lock, Travis CI failed when trying to execute
rspec:

  $ bundle install --deployment
  ...
  $ rspec
  /home/travis/build.sh: line 169: rspec: command not found

We believe that executing rspec in the context of the bundle will
resolve this.

Change-Id: Ie96202470068c610833436e2588d5c94d92b3e54
Reviewed-on: https://gerrit.causes.com/29841
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>