Add .es6.js to normalize_js_extension

[bouk]

@jejacks0n @mikepack

Some people use this as an extension for es6 files.

This might be better off as a configuration parameter, or alternatively you could look at Sprockets.mime_types["text/ecmascript-6"] to find out which extensions are registered to that mime type

[jejacks0n]

Can you confirm that the breakage isn't from your addition? It doesn't appear to be directly tied, but would appreciate your time.

[bouk]

@jejacks0n seemed to have been a fluke

[jejacks0n]

Thanks!