Use private lookup for these handles (#113)

publicLookup seems to have trouble when there are multiple versions of a
signature class in the classloader hierarchy.

Fixes #112

ext/java/org/jruby/ext/stringio/StringIO.java

@@ -1529,7 +1529,7 @@ public IRubyObject write(ThreadContext context, IRubyObject[] args) {
 
     static {
         MethodHandle cat, modify, substr;
-        MethodHandles.Lookup lookup = MethodHandles.publicLookup();
+        MethodHandles.Lookup lookup = MethodHandles.lookup();
         try {
             cat = lookup.findVirtual(RubyString.class, "catWithCodeRange", MethodType.methodType(RubyString.class, RubyString.class));
             modify = lookup.findVirtual(RubyString.class, "modifyAndClearCodeRange", MethodType.methodType(void.class));