Travis CI Build doesn't install gems for JRuby platform

I have a travis build set up for my project that also run on JRuby. I mention the activerecord-jdbcsqlite3-adapter gem in the Gemfile for the :jruby platform:

platforms :jruby do
  gem "activerecord-jdbcsqlite3-adapter"
end

but the build still always fails with the message LoadError: Please install the sqlite3 adapter:gem install activerecord-sqlite3-adapter(sqlite3 is not part of the bundle. Add it to Gemfile.) and the gem actually doesn't get installed.

The project is open source at http://ift.tt/1LVPBO8, the build is at http://ift.tt/1I97Z3q.