don't commit Gemfile.lock in html_text_helper gem

test plan: tests should pass

Change-Id: I148918fee298c0903b0449c187affaf19a553ddd
Reviewed-on: https://gerrit.instructure.com/32875
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>
QA-Review: Simon Williams <simon@instructure.com>