set encoding when reading from streams

using .toString() works most of the time but can lead to utf8 characters being
broken across chunk boundaries.

https://nodejs.org/api/stream.html#stream_readable_setencoding_encoding

app/js/CompileManager.js

@@ -334,7 +334,7 @@ module.exports = CompileManager = {
       proc.on('error', callback)
 
       let stderr = ''
-      proc.stderr.on('data', chunk => (stderr += chunk.toString()))
+      proc.stderr.setEncoding('utf8').on('data', chunk => (stderr += chunk))
 
       return proc.on('close', function(code) {
         if (code === 0) {