Add priv/test-for-gh-pr.sh, change TravisCI 'script' to use it

.travis.yml

@@ -1,6 +1,6 @@
 language: erlang
 notifications:
   email: scott@basho.com
-script: "make test"
+script: "priv/test-for-gh-pr.sh"
 otp_release:
   - 17.5

priv/test-for-gh-pr.sh

@@ -0,0 +1,10 @@
+#!/bin/sh
+
+set -x
+
+if [ "${TRAVIS_PULL_REQUEST}" = "false" ] then
+	echo '$TRAVIS_PULL_REQUEST is false, skipping tests'
+	exit 0
+else
+	make test
+fi