Upgrade to Babel 6

[sindresorhus]

https://babeljs.io/blog/2015/10/29/6.0.0/

We need to enforce Babel 6 (for devs wanting to use local version) as it's incompatible with 5.

[sindresorhus]

I think we should defer this for a couple of weeks until Babel 6 settles a bit. I see a lot of issues on the Babel issue tracker that might affect us.

[twada]

Just released babel-plugin-espower 2.0.0. Now power-assert works with Babel6!

[sindresorhus]

This was resolved in v0.9.0.