Replacing v4 JAR with v13 JAR

v13 is a superset of v4. In the original sample, the v13 jar was in the
src tree, which confused Eclipse. The PhoneGap push plugin readme
suggests that v13 should be used:
https://github.com/phonegap-build/PushPlugin
Merge pull request #40 from stevetraut/master

Replacing v4 JAR with v13 JAR