Add PHANTOMJS_VERSION env var & document in sample travis

sample_files/.travis.yml

@@ -33,6 +33,10 @@ addons:
 env:
   global:
   - VERSION="8.0" TESTS="0" LINT_CHECK="0" TRANSIFEX="0"
+  - PHANTOMJS_VERSION="latest"
+  # The above line controls the PhantomJS version that is used for JS testing.
+  #   It is not necessary to include this value unless you are altering the default.
+  #   Use `OS` to skip the PhantomJS upgrade & use the system version instead.
   - TRANSIFEX_USER='[email protected]'
   # This line contains the encrypted transifex password
   # To encrypt transifex password, install travis ruby utils with:

travis/travis_install_nightly

@@ -33,9 +33,11 @@ npm install -g less less-plugin-clean-css
 
 
 # Update PhantomJS (v10 compat)
-npm install --prefix ${TRAVIS_BUILD_DIR} [email protected]
-ln -s "${TRAVIS_BUILD_DIR}/node_modules/phantomjs-prebuilt/lib/phantom/bin/phantomjs" "${HOME}/maintainer-quality-tools/travis/phantomjs"
-
+if [ "${PHANTOMJS_VERSION}" != "OS" ];
+then
+    npm install --prefix ${TRAVIS_BUILD_DIR} "phantomjs-prebuilt@${PHANTOMJS_VERSION:=latest}"
+    ln -s "${TRAVIS_BUILD_DIR}/node_modules/phantomjs-prebuilt/lib/phantom/bin/phantomjs" "${HOME}/maintainer-quality-tools/travis/phantomjs"
+fi
 
 # For backward compatibility, take version from parameter if it's not globally set
 if [ "x${VERSION}" == "x" ] ; then