Set the file encoding correctly

[wdspkr]

This fixes end-of-input-errors that occur because of utf-8 characters in method arguments, as described here: #103 (comment)

[Arcanemagus]

What version of Ruby are you on, and what OS? The change this PR makes breaks setting the encoding properly on Windows at least, and doesn't match the documentation.

[Arcanemagus]

The fact that every single CI build failed also doesn't speak well for this being a good PR 😛.

[wdspkr]

So, this was the first try to do the obvious thing. But I guess we have to dig a little deeper.
The thing is, that in MacOS (Sierra) the commandline options are set differently. Here comes an excerpt of the ruby manpage in MacOS for ruby 2.3.3p222 (2016-11-21 revision 56859) [x86_64-darwin16]:

     -E external[:internal]
     --encoding external[:internal]
                    Specifies the default value(s) for external encodings and internal encoding. Values should be separated with colon (:).

                    You can omit the one for internal encodings, then the value (Encoding.default_internal) will be nil.

     --external-encoding=encoding
     --internal-encoding=encoding
                    Specify the default external or internal character encoding

Note the space between the -E flag and the encoding. 😞
But my change also stops linter-ruby from working.

I tried around a little more, can't replicate the wrong behaviour in the test and might just refrain to always starting atom from the command line (as the encoding is set then correctly by the environment).

[Arcanemagus]

Ah ha! It looks like the --external-encoding=encoding and --internal-encoding=encoding options are the same, and the -Eex (-E ex) option is just an alias to setting both to the specified value. So if we move to specifying them individually it should work everywhere!