Update TravisCI to use exiting env var

$TRAVIS_COMMIT_RANGE was broken but it seems what we're doing is worse and
resulting in false negatives.

The result of the bad range was that we weren't running lint or tests for
things we should have been. It actually looks like $TRAVIS_COMMIT has been
wrong and it's not clear why this has been working at all.

.travis.yml

@@ -8,10 +8,6 @@ cache:
   - node_modules
 before_install:
 - |
-    if [ "$TRAVIS_PULL_REQUEST" != "false" ]; then
-      PR_FIRST=$(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."
       exit