Lock jquery-ui-rails to a version up to 4.2.1

jquery-ui-rails changed the way one must reference a module in their manifests
in v5.0.0 from a `.` to a `/`. Since base.js.coffee uses the dot syntax, we
should set the upper limit of the gem version for now.

See:
https://github.com/joliss/jquery-ui-rails/commit/774f8b3e2e5ae8b80515128dd0cdb5cde48dfc01