tools: do not lint commit message if var undefined
Check that $TRAVIS_COMMIT_RANGE is set before trying to lint commit messages in Travis CI.
Refs: https://github.com/nodejs/node/pull/23572#issuecomment-430850382 Refs: https://github.com/nodejs/node/pull/22842#issuecomment-430850049
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
commit message follows commit guidelines