Be explicit about ECMAScript version of Gulpfile

If Node tries to parse the Gulpfile as the wrong version of ECMAScript it will
choke.

This can be stopped by explicitly telling it what version of ECMAScript to use,
as suggested here: https://markgoodyear.com/2015/06/using-es6-with-gulp/

Fixes #74

.babelrc

@@ -0,0 +1,3 @@
+{
+  "presets": ["es2015"]
+}