Relay should use babel.parse instead of babylon perhaps

At the moment using Babylon to parse files leaves relay always playing catch up with adding syntax plugins, and doesn't handle the mutually exclusive cases at all (typescript vs flow, etc).

Babel just added [`babel.parse`](https://github.com/babel/babel/tree/master/packages/babel-core#babelparsecode-string-options-object) to return the ast, but critically, will use the local babelrc if it exists. Which means parsing will always be using the set of plugins the user has defined for that project.