Automate version changes

[bitjson]

We need to be sure to increment the version in config.xml as well as package.json, since cordova seems to add that version as the spec when the plugin is added via:

cordova plugin add cordova-plugin-qrscanner --save

Right now, the above results in the following being added to config.xml, even though the currently deployed version on npm is 2.3.4:

<plugin name="cordova-plugin-qrscanner" spec="~2.2.0" />

It would be good to add something to the prep-release npm script to copy the package.json version over to config.xml.