Upgrade babel to version 7

[realityking]

No functional difference but it generates somewhat nicer unminified code.

[avoidwork]

thanks!

[avoidwork]

Re-adding comment.

Had to force push it out due to:

Running "babili" task
Warning: Cannot find module 'babel-core'
Require stack:
- C:\Users\jason\Projects\filesize.js\Gruntfile.js
- C:\Users\jason\Projects\filesize.js\node_modules\grunt\lib\grunt\task.js
- C:\Users\jason\Projects\filesize.js\node_modules\grunt\lib\grunt.js
- C:\Users\jason\Projects\filesize.js\node_modules\grunt-cli\bin\grunt
- C:\Users\jason\Projects\filesize.js\node_modules\grunt\bin\grunt Use --force to continue.

Aborted due to warnings.

This project needs a tool chain update.

[realityking]

Oh damn, sorry about that. I do have another PR almost ready to use rollup as the build system. I'll make this commit part of that branch.