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/LocalCommandRunner.js

@@ -46,7 +46,7 @@ module.exports = CommandRunner = {
     const proc = spawn(command[0], command.slice(1), { cwd: directory, env })
 
     let stdout = ''
-    proc.stdout.on('data', data => (stdout += data))
+    proc.stdout.setEncoding('utf8').on('data', data => (stdout += data))
 
     proc.on('error', function(err) {
       logger.err(