Check JRuby version without accessing Gem

When running with --disable-gems, JRuby still needs to be able to
load FFI from stdlib. Accessing the Gem namespace here requires
that gems not be disabled.

This patch uses a simple split and integer comparison to compare
the first two digits of the JRuby version with 9.3.

See ffi#763 for the original code.

lib/ffi.rb

@@ -8,7 +8,7 @@
 
   require 'ffi/ffi'
 
-elsif RUBY_ENGINE == 'jruby' && Gem::Version.new(RUBY_ENGINE_VERSION) >= Gem::Version.new("9.3.pre")
+elsif RUBY_ENGINE == 'jruby' && RUBY_ENGINE_VERSION.split('.').yield_self {|maj, min| maj.to_i >= 9 && min.to_i >= 3}
   JRuby::Util.load_ext("org.jruby.ext.ffi.FFIService")
   require 'ffi/ffi'