chore(presubmit): correct condition

Author: mhevery

scripts/travis/presubmit.sh

@@ -13,7 +13,7 @@ echo Test result is: $TRAVIS_TEST_RESULT
 
 if [ "$CHANNEL" = "stable" ] && [ "$TRAVIS_REPO_SLUG" = "angular/angular.dart" ]; then
 
-    if [ $TRAVIS_TEST_RESULT -eq 0 ] && [ $TRAVIS_BRANCH = presubmit-* ]; then
+    if [ $TRAVIS_TEST_RESULT -eq 0 ] && [[ $TRAVIS_BRANCH == "presubmit-"* ]]; then
 
         git config credential.helper "store --file=.git/credentials"
         echo "https://${GITHUB_TOKEN_ANGULARDART}:@github.com" > .git/credentials