[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