Lock beaker version to ~> 1.17.0

If beaker is installed with no version specified it will now install
beaker 2.0.0 - normally that would be fine, but it upgrades the
dependencies so that mime-types 2.4.3 gets installed, which isn't
compatible with Ruby <= 1.9.3

By locking the beaker version to ~> 1.17.0 (currently 1.17.6) we install
mime-types 1.25.1 which is compatible with all the versions of ruby we
currently want to test.