lock webmock version to 1.19.x
lock webmock version to 1.19.x

It appears that the webmock API changed in version 1.20.0 such that
hashes are no longer accepted as arguments in to_return, which causes
several test failures. For now, force webmock to a working version to
ensure that tests pass. This only affects development.

dwradcliffe/groupme,ahamlinman/groupme