Remove beaker from Gemfile

Having both `beaker` and `beaker-rspec` in the Gemfile results
in a dual inclusion of `beaker` as a dependency resulting in
version 2.0.0 of `beaker-rspec` being installed.

The tests for vcsrepo require the `BEAKER_setfile` ENV var, which
is only available in `beaker-rspec` 2.2.0 and above. Removing
`beaker` allows the latest version of `beaker-rspec` to be
installed, thus satisfying this requirement.