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.

Change the jenins.sh to update the version of the submodule as part of jenkins.sh and commit the updated submodule after the gem has been published. I can then update jenkins to push the master branch as a post build step.
Another thought: this will create a new commit for every build, even when the version hasn't changed.  We might want to change this so that is uses logic similar to govuk_template (see https://github.com/alphagov/govuk_template/blob/master/Rakefile#L66-L96)