Add listen v3.0.x as a development dependency
Add listen v3.0.x as a development dependency

Since v3.1.0 `listen` depends on Ruby v2.x. For Travis CI we use the lowest possible version of Ruby which Rails supports. So the intention of this commit is to keep syncing the lowest possible Ruby version and the version of `listen`.
tmiyamon/acts-as-taggable-array-on