[TEST] Execute tests in the `test:all` Rake task manually, to prevent dependency loading errors

Should prevent errors like:

    .../elasticsearch-rails/elasticsearch-rails/test/test_helper.rb:21:in `require': cannot load such file -- rails/version (LoadError)
      from .../elasticsearch-rails/elasticsearch-rails/test/test_helper.rb:21:in `<top (required)>'

See:

* https://api.travis-ci.org/jobs/18002966/log.txt?deansi=true
* http://build.elasticsearch.com/view/Clients/job/es-rails_core/ES_V=1.x_nightly,GEMFILE=4.0.gemfile,RUBY_V=2.0.0-p247,label=not-es/lastCompletedBuild/consoleText