require FileReader in FileWriter to fix 'write' exec not being called (apache#237)

FileWriter is missing the require('./FileReader') module causing the
native FileReader to be used instead of the FileReader proxy.

Oddly this does not happen everytime, but I could consistently
reproduce the issue when running subsequent `cordova run android`
commands on an Ionic app running in a 7.1.1 Android Emulator.

Author: wfairclough

www/FileWriter.js

@@ -21,6 +21,7 @@
 
 var exec = require('cordova/exec');
 var FileError = require('./FileError');
+var FileReader = require('./FileReader');
 var ProgressEvent = require('./ProgressEvent');
 
 /**