Remove calls to JavaUtil.convertRubyToJava

JavaUtil.convertRubyToJava is [deprecated][1]. This commit replaces it
with calls to more specific methods which perform necessary conversions.

[1]: http://jruby.org/apidocs/org/jruby/javasupport/JavaUtil.html#convertRubyToJava%28org.jruby.runtime.builtin.IRubyObject%29