Change surround variable TRAVIS_BRANCH with double-quotes instead of single-quotes, to get interpreted correctly in dev/travis/before_script.sh

Author: adrian-martinez-interactiv4

dev/travis/before_script.sh

@@ -71,7 +71,7 @@ case $TEST_SUITE in
             --output-file="$changed_files_ce" \
             --base-path="$TRAVIS_BUILD_DIR" \
             --repo='https://github.com/magento/magento2.git' \
-            --branch='$TRAVIS_BRANCH'
+            --branch="$TRAVIS_BRANCH"
         cat "$changed_files_ce" | sed 's/^/  + including /'
 
         cd ../../..