Updating mixlib-shellout gem

Pinned version (1.6.0) of ```mixlib-shellout``` gem which we were using
was removed from public rubygems when the latest update was published so
we had to update ```Gemfile.lock```. We can only hope this won't happen
again :-)
Merge pull request #14 from ministryofjustice/bundle-fix

Updating mixlib-shellout gem