Get original repository branch name

Travis does not provide the original repository branch name on any
environment variable. We are getting the information from the GitHub
API instead.

The misleading environment variable `TRAVIS_BRANCH` returns `master`
instead:
> TRAVIS_BRANCH: For builds not triggered by a pull request this is the
name of the branch currently being built; whereas for builds triggered
by a pull request this is the name of the branch targeted by the pull
request (in many cases this will be master).

Reference: http://docs.travis-ci.com/user/environment-variables/

.travis.yml

@@ -9,6 +9,7 @@ env:
     - OMF_REPO_BRANCH="$TRAVIS_BRANCH"
 
 before_install:
+  - export OMF_REPO_BRANCH=`tools/branch-name.sh`
   - docker build -t fish . < Dockerfile
 
 before_script:

tools/branch-name.sh

@@ -0,0 +1,16 @@
+#!/usr/bin/env sh
+
+URL=https://api.github.com/repos/$TRAVIS_REPO_SLUG/pulls/$TRAVIS_PULL_REQUEST
+
+BRANCH_NAME=`curl -s $URL ^/dev/null \
+    | grep -C 2 "head" \
+    | grep \"ref\" \
+    | cut -d':' -f2- \
+    | sed -e 's/["|,]//g;s/^[ \t]//g'
+  `
+
+if [ -n "$BRANCH_NAME" ]; then
+  echo $BRANCH_NAME
+else
+  echo $TRAVIS_BRANCH
+fi