Update jar-dependencies

We've decided that the hard dependency on ruby-maven is too large
and too sticky to go with, so I've reverted that change and we will
continue to work through the parallel-install issues separately.

See ruby/rubygems#9386 (comment)

I also modified stringio to no longer include ruby-maven as a
dev dependency, since it does not need it. This dodges the issue
for now by avoiding the ruby-maven dependencies being installed by
both bundler and the post-install jar-dependencies hook.

Author: headius

lib/pom.rb

@@ -48,7 +48,7 @@ def log(message = nil)
   # ['io-nonblock', '0.3.2'],
   ['io-wait', '0.4.0'],
   ['ipaddr', '1.2.8'],
-  ['jar-dependencies', '0.5.6'],
+  ['jar-dependencies', '0.5.7'],
   ['jruby-readline', '1.3.7'],
   ['jruby-openssl', '0.15.5'],
   ['json', '2.18.0'],