[MODEL] Prevent unit test failures under ActiveRecord 3.x

Added missing stub for `exec_queries`.
Added the commonly used gems to the main Gemfile
Removed the invocation of `elasticsearch:update` from the main Rakefile
Allow passing specific BUNDLE_GEMFILE from the main `test:*` Rake tasks
[MODEL] Renamed the 3.0 and 4.0 gemfiles for ActiveRecord and Mongoid
Added installing the 3.0 and 4.0 gemfiles to the main `rake bundle:install` task
Added removing the 3.0 and 4.0 Gemfiles to the main `rake bundle:clean` Rake tasks
[CI] Added 3.0 and 4.0 to the Travis matrix
[CI] Launch MongoDB before the test runs on Travis
[CI] Use the stock Elasticsearch pre-installed by Travis
[CI] Replaced `gemfile` specification for Travis with custom `TEST_BUNDLE_GEMFILE` environment variable

Run tests like this locally:

    $ SERVER=start TEST_BUNDLE_GEMFILE=$PWD/elasticsearch-model/gemfiles/3.0.gemfile TEST_CLUSTER_COMMAND=/tmp/elasticsearch-1.0.0.RC1/bin/elasticsearch bundle exec rake test:all

    $ SERVER=start TEST_BUNDLE_GEMFILE=$PWD/elasticsearch-model/gemfiles/4.0.gemfile TEST_CLUSTER_COMMAND=/tmp/elasticsearch-1.0.0.RC1/bin/elasticsearch bundle exec rake test:all