Update manifestWriter.js to fix #133 in Cordova 6+

[brnosouza]

It should work with #133 on Cordova 6+... I keept the old path for cordova old versions and checked if it exists

[Twister42]

Please merge this and decline PR #135 so we can use your plugin with cordova 6+ :)

[Twister42]

var fs = require('fs'); is missing at the top of the file, I did a new one with everything #157

[brnosouza]

@Twister42 thx, I didn't see its missing. I will close my PR in favor of yours #157