Encoding error while calling execjs

Environments:

- docker image based on `ruby:2.3`
- ruby-babel-transpiler 2.7.0

I'm faced with an encoding issue when using `jekyll-babel` which depends on `ruby-babel-transpiler`:

```
root@1ac6426859a3:/data# ( cd /data/ && jekyll build -t )
Configuration file: /data/_config.yml
Source: /data
Destination: /data/_site
Incremental build: disabled. Enable with --incremental
Generating...
Conversion error: Jekyll::Converters::Babel encountered an error while converting 'static/js/help.es6':
"\xEF" on US-ASCII
/usr/local/bundle/gems/execjs-2.7.0/lib/execjs/encoding.rb:22:in `encode': "\xEF" on US-ASCII (Encoding::InvalidByteSequenceError)
from /usr/local/bundle/gems/execjs-2.7.0/lib/execjs/encoding.rb:22:in `encode'
from /usr/local/bundle/gems/execjs-2.7.0/lib/execjs/external_runtime.rb:8:in `initialize'
from /usr/local/bundle/gems/execjs-2.7.0/lib/execjs/runtime.rb:57:in `new'
from /usr/local/bundle/gems/execjs-2.7.0/lib/execjs/runtime.rb:57:in `compile'
from /usr/local/bundle/gems/execjs-2.7.0/lib/execjs/module.rb:27:in `compile'
from /usr/local/bundle/gems/babel-transpiler-0.7.0/lib/babel/transpiler.rb:24:in `context'
from /usr/local/bundle/gems/babel-transpiler-0.7.0/lib/babel/transpiler.rb:28:in `transform'
from /usr/local/bundle/gems/jekyll-babel-1.1.0/lib/jekyll/converters/babel.rb:24:in `convert'
from /usr/local/bundle/gems/jekyll-3.3.1/lib/jekyll/renderer.rb:109:in `block in convert'
from /usr/local/bundle/gems/jekyll-3.3.1/lib/jekyll/renderer.rb:107:in `each'
from /usr/local/bundle/gems/jekyll-3.3.1/lib/jekyll/renderer.rb:107:in `reduce'
from /usr/local/bundle/gems/jekyll-3.3.1/lib/jekyll/renderer.rb:107:in `convert'
from /usr/local/bundle/gems/jekyll-3.3.1/lib/jekyll/renderer.rb:86:in `run'
from /usr/local/bundle/gems/jekyll-3.3.1/lib/jekyll/site.rb:463:in `block in render_pages'
from /usr/local/bundle/gems/jekyll-3.3.1/lib/jekyll/site.rb:461:in `each'
from /usr/local/bundle/gems/jekyll-3.3.1/lib/jekyll/site.rb:461:in `render_pages'
from /usr/local/bundle/gems/jekyll-3.3.1/lib/jekyll/site.rb:191:in `render'
from /usr/local/bundle/gems/jekyll-3.3.1/lib/jekyll/site.rb:69:in `process'
from /usr/local/bundle/gems/jekyll-3.3.1/lib/jekyll/command.rb:26:in `process_site'
from /usr/local/bundle/gems/jekyll-3.3.1/lib/jekyll/commands/build.rb:63:in `build'
from /usr/local/bundle/gems/jekyll-3.3.1/lib/jekyll/commands/build.rb:34:in `process'
from /usr/local/bundle/gems/jekyll-3.3.1/lib/jekyll/commands/build.rb:16:in `block (2 levels) in init_with_program'
from /usr/local/bundle/gems/mercenary-0.3.6/lib/mercenary/command.rb:220:in `block in execute'
from /usr/local/bundle/gems/mercenary-0.3.6/lib/mercenary/command.rb:220:in `each'
from /usr/local/bundle/gems/mercenary-0.3.6/lib/mercenary/command.rb:220:in `execute'
from /usr/local/bundle/gems/mercenary-0.3.6/lib/mercenary/program.rb:42:in `go'
from /usr/local/bundle/gems/mercenary-0.3.6/lib/mercenary.rb:19:in `program'
from /usr/local/bundle/gems/jekyll-3.3.1/exe/jekyll:13:in `'
from /usr/local/bundle/bin/jekyll:17:in `load'
from /usr/local/bundle/bin/jekyll:17:in `'
```

After hours of debuging, I find that the problem is at [here](https://github.com/babel/ruby-babel-transpiler/blob/6878f6edb4e717066bbe344c25b05055cd2d914f/lib/babel/transpiler.rb#L24):

```
def self.context
@context ||= ExecJS.compile("var self = this; " + File.read(script_path))
end
```

It reads `babel.js` with `File.read` directly without any paramters, which return a string with encoding `US-ASCII`, while the correct encoding is `UTF-8` indeed. Then `execjs` treats the string as a `US-ASCII` encoded string and trys to transcode it to `UTF-8` so it fails because of non-ASCII characters.

The solution is to add a parameter to `File.read` according to the [doc](http://ruby-doc.org/core-2.3.0/IO.html#method-c-read):

```
def self.context
script = File.read(script_path, :encoding => 'UTF-8')
@context ||= ExecJS.compile("var self = this; " + script)
end
```

I have little knowledge about ruby, please correct me if there are some mistakes.