Fix check for commit ranges in racing PRs.

See facebook#1768 / facebook#2000 for original issue.
See travis-ci/travis-ci#1719 (comment) for problem description and origin of this fix.
This is still a temporary fix until travis-ci/travis-core#383 is merged.

Author: RReverser

.travis.yml

@@ -8,7 +8,8 @@ cache:
 before_install:
 - |
     if [ "$TRAVIS_PULL_REQUEST" != "false" ]; then
-      TRAVIS_COMMIT_RANGE="FETCH_HEAD...$TRAVIS_BRANCH"
+      PR_FIRST=$(curl -s https://github.com/${TRAVIS_REPO_SLUG}/pull/${TRAVIS_PULL_REQUEST}.patch | head -1 | grep -o -E '\b[0-9a-f]{40}\b' | tr -d '\n')
+      TRAVIS_COMMIT_RANGE=$PR_FIRST^..$TRAVIS_COMMIT
     fi
     git diff --name-only $TRAVIS_COMMIT_RANGE | grep -qvE '(\.md$)|(^(docs|examples))/' || {
       echo "Only docs were updated, stopping build process."