Resolves Java extension compilation problem on OSX

The changes introduced by 2e2b204804108c5c53284bb48cf6c06ac553d120
has an incompatibiliy with Java on OSX. This makes unable to compile
Java extension on OSX.
From the source code, the second argument of JAXPExtensionsProvider
constructor is false by default and we don't need it.
Luckily, a single argument constructor of JAXPExtensionsProvider
exists on OSX Java. This change eliminates the second argument.

Also, source/target version to 1.6 are added to rake compiler.