Bump to 1.6.1 and use version from submodule

Use the version from the submodule for the gem version. So it is
consistent with other types of toolkit packages.
Update submodule and commit after gem publish

This lets us set the jenkins job for this repo as a post commit hook to
the govuk_frontend_toolkit repo. It will ensure the gem is always up to
date with that repository.

Bumping the gem will then be a fully automatic process controlled by the
govuk_frontend_toolkit repo.
Merge pull request #8 from alphagov/version-from-submodule

Bump to 1.6.1 and use version from submodule