Fails with an unlabelled error on Windows

On my Windows 10 64-bit it fails with:

```
Conversion error: Jekyll::Converters::Babel encountered an error while converting 'assets/js/components/PlayButton.es6':

(execjs):1: ExecJS::RuntimeError
from C:/Ruby22-x64/lib/ruby/gems/2.2.0/gems/execjs-2.7.0/lib/execjs/external_runtime.rb:184:in `exec_runtime'
from C:/Ruby22-x64/lib/ruby/gems/2.2.0/gems/execjs-2.7.0/lib/execjs/external_runtime.rb:39:in `exec'
from C:/Ruby22-x64/lib/ruby/gems/2.2.0/gems/execjs-2.7.0/lib/execjs/external_runtime.rb:14:in `initialize'
from C:/Ruby22-x64/lib/ruby/gems/2.2.0/gems/execjs-2.7.0/lib/execjs/runtime.rb:57:in `new'
...
```

After banging my head against the wall for a couple of hours I figured out this is the cause (and solution): https://github.com/babel/ruby-babel-transpiler/pull/295

So until that's pulled and included in the next release it'll most likely be broken on Windows.

Start with the conversion of assets/js/components/PlayButton.es6 and the ExecJS stack trace; review the referenced ruby-babel-transpiler pull request and the dependency version used by this repository. Done means the Windows conversion no longer fails with the reported error after a compatible release is included.

Written by the indexing model from the issue text.