Bullet Wrapper compilation outdated documentation

[yahel-ck]

Issue details

The Wiki contains outdated documentation on how to compile the bullet wrapper here.

The doc says the native library should be compiled like so:

cd libgdx/extensions/gdx-bullet/jni
ant -f build.xml all

However libgdx/extensions/gdx-bullet/jni/build.xml was deleted in this PR.

The PR says the newer way to compile it is to use the gradle task jnigen/jnigenBuild but I couldn't get it to work yet.
The builds were successfull but it seems like my CPP changes weren't actually compiled in (I got a No implementation found exception for my new JNI method).

If anyone knows what's the correct way to compile the wrapper please let me know!
thanks

[crykn]

@PokeMMO, @Berstanio: could one of you update the docs?