Fix #1664 for Hash#compare_by_identity and Hash#store

https://github.com/jruby/jruby/issues/1664

Test in RubySpec:
https://github.com/rubyspec/rubyspec/pull/274
Merge pull request #1666 from godfat/jruby-1_7-no-copy-for-compare_by_identity

Fix #1664 for Hash#compare_by_identity and Hash#store
allow to build jruby distribution with default 1.8 compatibility
missed to configure the plugin repo as well
fixed issue 1517