do not package .babelrc

Having this file in the dependency tree of my application breaks the build because it introduces config invalid for it (I am using Rollup with Babel). I don't think this file is necessary in the NPM package because the code is published already transpiled, so I think it's best to `.npmignore` it.

.npmignore

@@ -1,3 +1,4 @@
 src/
 .idea/
-test/
+test/
+.babelrc