Add Gemfile.lock back to .gitignore
Add Gemfile.lock back to .gitignore

This was mistakenly removed in a prior commit. As per the conventions of
these frontend engines, ignore the lock file and rely on the gemspec to
declare a loose set of version constraints around dependencies.

moneyadviceservice/mortgage_calculator,moneyadviceservice/mortgage_calculator,moneyadviceservice/mortgage_calculator,moneyadviceservice/mortgage_calculator