Add babel-plugin-transform-async-to-generator for Node 6 compatibility

[kohlmannj]

Hi,

I saw the deasync branch / PR (#2), however, I'm not sure I can lend any expertise to that discussion. However, I do need to use this plugin in a Node v6 environment, which does not natively support async / await.

Hence, as an apparent stopgap, I have added babel-plugin-transform-async-to-generator to .babelrc, which appears to generate a build which works on Node v6.

[coveralls]

Coverage remained the same at 100.0% when pulling 947ee0a on kohlmannj:master into 55679c2 on wbyoung:master.

[wbyoung]

We should just change this line instead to be 6.10.11 and continue to rely on preset-env.

Please try that and open a new PR that also updates the build targets. Thanks for contributing!