Remove use of deprecated org.jruby.Ruby.fastGetModule

The `fastGetModule` is deprecated in jruby 9.4 and removed in 10:

https://github.com/jruby/jruby/blob/1611c8e4e464164474a0e8c0663e16d67fa5c268/core/src/main/java/org/jruby/Ruby.java#L1373-L1385

This commit uses the getModule method (which should bridge between 9 and 10).
Note that it is marked deprecated in 10 but having the 9/10 compatability will
be a win in the forseable future.

https://github.com/jruby/jruby/blob/02c56b6099582500f86262a1d16d6b0e5cf3fe95/core/src/main/java/org/jruby/Ruby.java#L1339-L1342

Author: donoghuc

src/main/org/bson_ruby/NativeService.java

@@ -56,7 +56,7 @@ public class NativeService implements BasicLibraryService {
    * @since 2.0.0
    */
   public boolean basicLoad(final Ruby runtime) throws IOException {
-    RubyModule bson = runtime.fastGetModule(BSON);
+    RubyModule bson = runtime.getModule(BSON);
     GeneratorExtension.extend(bson);
 
     RubyClass byteBuffer = bson.defineClassUnder("ByteBuffer", runtime.getObject(), new ObjectAllocator() {