Fix Travis for PRs against non-master (like release, for example)

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)

.travis.yml

@@ -13,8 +13,13 @@ before_script:
   - sudo apt-get update -qq
   - sudo apt-get install -qq python-yaml
   - git remote add upstream git://github.com/dotcloud/docker.git
-  - git fetch --append --no-tags upstream refs/heads/master:refs/remotes/upstream/master
+  - upstream=master;
+    if [ "$TRAVIS_PULL_REQUEST" != false ]; then
+      upstream=$TRAVIS_BRANCH;
+    fi;
+    git fetch --append --no-tags upstream refs/heads/$upstream:refs/remotes/upstream/$upstream
 # sometimes we have upstream master already as origin/master (PRs), but other times we don't, so let's just make sure we have a completely unambiguous way to specify "upstream master" from here out
+# but if it's a PR against non-master, we need that upstream branch instead :)
 
 script:
   - hack/travis/dco.py

hack/travis/env.py

@@ -6,7 +6,7 @@ if 'TRAVIS' not in os.environ:
 	exit(127)
 
 if os.environ['TRAVIS_PULL_REQUEST'] != 'false':
-	commit_range = [os.environ['TRAVIS_BRANCH'], 'FETCH_HEAD']
+	commit_range = ['upstream/' + os.environ['TRAVIS_BRANCH'], 'FETCH_HEAD']
 else:
 	try:
 		subprocess.check_call([