parsing versionCode doesn't respect 32-bit ints

[tjlahr]

The routine that's parsing the compiled manifest isn't handling 0xffffffff correctly. If I set versionCode to -1 in my AndroidManifest.xml, ruby_apk parses this as 4294967295.

This probably requires a fix in axml_parser.rb, but I haven't had a chance to track down exactly where yet.