Expand DistZip lib search

Previously, the DistZip container only searched for a JARs in the lib/
directory; it did not search recursively.  A user has now reported that this
isn't working for their application.  This change enlarges that search to be
recursive which should solve the issue.

[resolves #332]

Author: nebhale

lib/java_buildpack/container/dist_zip.rb

@@ -56,7 +56,7 @@ def supports?
       private
 
       def jars?
-        (lib_dir + '*.jar').glob.any?
+        (lib_dir + '**/*.jar').glob.any?
       end
 
       def lib_dir