Revert BUNDLED_WITH to 1.15.1
Revert BUNDLED_WITH to 1.15.1

We are still pinnned to 1.15.1 by puppet even though 1.15.4 is out, and
it's better to keep the Gemfile.lock in sync with the real versions on
our deployed environments to avoid any unexpected output.

alphagov/frontend,alphagov/frontend,alphagov/frontend,alphagov/frontend