circleci: Disable gitlint for now.

We get the following error (edited slightly):

  Dec 19 06:13:27 commit_messages| An error occurred while executing
    '/usr/bin/git rev-list --max-count=-1 upstream/master..HEAD':
  fatal: ambiguous argument 'upstream/master..HEAD':
    unknown revision or path not in the working tree.
  Use '--' to separate paths from revisions, like this:
  'git <command> [<revision>...] -- [<file>...]'

We'll need to adjust the remotes to make `upstream` mean what we expect.

tools/travis/backend

@@ -6,7 +6,7 @@ echo "Test suite is running under $(python --version)."
 set -e
 set -x
 
-./tools/lint --backend
+./tools/lint --backend ${CIRCLECI:+--no-gitlint}
 ./tools/test-tools
 ./tools/test-backend --coverage