.travis.yml: fetch base-branch for rebase

.travis.yml

@@ -8,6 +8,10 @@ services:
 before_install:
   - docker pull riot/riotbuild
 
+before_script:
+  # Fetch the base branch when it is not `master`
+  - git fetch origin "${TRAVIS_BRANCH}:${TRAVIS_BRANCH}"
+
 script:
   - docker run -a STDIN -a STDOUT -a STDERR --rm -u "$(id -u)"
       -v "${PWD}:/data/riotbuild" -v /etc/localtime:/etc/localtime:ro