Remove test_project's Gemfile.lock

Also update the specs to not rely on specific packaged gem versions
Hack to ensure test_project uses default gems when available

This should ensure that a test run under JRuby 1.7.4 would download the
latest json gem as a dependency, whereas a run under JRuby 1.7.5 would
use the JRuby-bundled versions of json and jruby-openssl
Support default gems with no directory under gems

Rubygems 2.1 introduced a default gem feature and JRuby >= 1.7.5 uses
that feature to bundle some gems within the standard library gem path.

As the gems are bundled inside jruby-complete.jar, we do not need to add
them once more to the jar file. Trying to do so actually produces an
exception, since there is no gem directory for them.

This should resolve issue #15.