Use the Enforce plugin to ensure our dependencies are compatible with Java 5

[GoogleCodeExporter]

Use the Enforce Bytecode Version Plugin to ensure our dependencies are 
compatible with Java 5:

http://mojo.codehaus.org/extra-enforcer-rules/enforceBytecodeVersion.html

The java6 package should obviously enforce Java 6 compatibility. App Engine 
also requires at least Java 6.

We attempted to do this once, but ran into the following problem:

The Enforcer plugin crashes with a NullPointerException. This seems to be 
because we specify version numbers in <dependencyManagement> instead of 
directly on the dependencies. 

The following thread appears to be the issue we're facing as well: 
http://stackoverflow.com/questions/1017019/maven-assembly-plugin-fails-with-null
pointerexception 
Particularly the comment on the answer:
http://stackoverflow.com/questions/1017019/maven-assembly-plugin-fails-with-null
pointerexception#comment826448_1017037

Original issue reported on code.google.com by ngmic...@google.com on 2 Jul 2013 at 4:06