gem install failure on Windows

Windows 7
jruby 9.0.0.0.pre1 (2.2.0p0) 2015-01-20 d537cab Java HotSpot(TM) Client VM 24.51-b03 on 1.7.0_51-b13 +jit [Windows 7-x86]

```
C:\>jruby -S gem install test-unit
ERROR:  Loading command: install (Win32::Registry::Error)
The system cannot find the file specified.
ERROR:  While executing gem ... (NoMethodError) undefined method `invoke_with_build_args' for nil:NilClass
```
