Fix encoding specification

With the space in there some platforms treat the encoding as ` utf-8`
instead of the desired `utf-8`. Since the `ruby` help text shows no space
in there in the first place, this is more correct and works on platforms
where it was broken before.

Author: Arcanemagus

lib/main.js

@@ -36,7 +36,7 @@ export default {
           return [];
         }
 
-        const execArgs = ['-wc', '-E utf-8'];
+        const execArgs = ['-wc', '-Eutf-8'];
         const execOpts = {
           stdin: fileText,
           stream: 'stderr',