Actually use Java7ClassValue when using Java 7+.

When f5850a515adadc6589cc286d4d38c2fecc79f2bf was created, the `org.jruby.java.proxies.ClassValueProxyCache` class was renamed as `org.jruby.util.collections.Java7ClassValue`. But that class was only referenced via reflection, and so IDE-based refactoring did not catch it, and `Java7ClassValue` ended up being totally unused.