Platform's version should be defined in only one place

Currently every platform has a multiple places where the version is kept and should be updated on every version bump. E.g. for android platform these places are:

• package.json
• VERSION plain-text file at the root of the platform's package
• version executable script in bin/templates/cordova/version (copied to cordova/version after platform is added to the project)
• CORDOVA_VERSION variable in framework/src/org/apache/cordova/CordovaWebView.java

This variety of version sources is becoming too hard to maintain and may lead to various errors in released packages.

The proposed approach is to store version in package.json

References:

• coho updateRepoVersion method
• instructions on how to update platform's versionbefore release