fix(travis): validate TRAVIS_COMMIT if TRAVIS_PULL_REQUEST_SHA is not set. (#3094)

Looks like the pre-merge travis run sets TRAVIS_PULL_REQUEST_SHA but the post-merge run does not.
In the post-merge the TRAVIS_COMMIT should be the PR, so validate it.

Author: johnjbarton

.travis.yml

@@ -36,10 +36,11 @@ before_script:
   - sh -e /etc/init.d/xvfb start
   - npm run init
   - export $(openssl aes-256-cbc -pass env:CREDENTIALS_PASS -d -in credentials)
-  - echo "git head commit $(git rev-parse FETCH_HEAD)"
   - echo "TRAVIS_COMMIT $TRAVIS_COMMIT"
   - echo "TRAVIS_PULL_REQUEST_SHA $TRAVIS_PULL_REQUEST_SHA"
-  - 'if [ "$VALIDATE_COMMIT_MSG" == "true" ]; then ./scripts/validate-commit-msg.sh $TRAVIS_PULL_REQUEST_SHA; fi'
+  - COMMIT_TO_VALIDATE=${TRAVIS_PULL_REQUEST_SHA:-TRAVIS_COMMIT}
+  - echo "COMMIT_TO_VALIDATE $COMMIT_TO_VALIDATE"
+  - 'if [ "$VALIDATE_COMMIT_MSG" == "true" ]; then ./scripts/validate-commit-msg.sh $COMMIT_TO_VALIDATE; fi'
   - 'if [ "$LINT" == "true" ]; then npm run lint; fi'
 
 script: