Add cache:bundler mention
Add cache:bundler mention

See https://travis-ci.org/metricfu/metric_fu/builds/15048354
and
https://github.com/metricfu/metric_fu/commit/5de0ad9e9a18cabd57fb6181adb28402d9e220bb

bf4/bf4.github.com,bf4/bf4.github.com,bf4/bf4.github.com,bf4/bf4.github.com