Inspect ruby version instead of jar file

From https://github.com/heroku/heroku-buildpack-java/blob/b3c51f94de6a43bc656b777bc84ef2a9451371aa/test/spec/java_spec.rb#L78

> OpenJDK versions > 9 do not have the jre/lib/ext directory where we drop the pgconfig.jar

This test was added by Joe in 248b30a later I moved to not having a separate track for installing JVM 648a149 so I removed the logic for installing a pgconfig.jar and relied on the buildpack. According to the JVM buildpack it's no longer in this updated version.

It seems the original PR was using this to test that installation logic. While we no longer need to do that (as the logic is part of the `heroku/jvm` buildpack) I wanted to preserve calling a `heroku run` to assert something about the dyno that would indicate that jruby was installed as we expect.

spec/hatchet/rubies_spec.rb

@@ -54,7 +54,7 @@
         app.push!
         expect(app.output).to match("JRUBY_OPTS is: --dev")
 
-        expect(app.run("ls .jdk/jre/lib/ext/")).to match("pgconfig.jar")
+        expect(app.run("ruby -v")).to match("jruby")
       end
     end
   end