Update to Jython 2.7-b3

It sounds like Jython 2.5.3 is leaking some threads.

Jython 2.5.4.rc1 has the same issue.

Jython 2.7-b3 fixes it.

Typical error when running tests:

```
ERROR   0.00s J2 | PythonScriptEngineTests (suite) <<<
   > Throwable #1: com.carrotsearch.randomizedtesting.ThreadLeakError: 1 thread leaked from SUITE scope at org.elasticsearch.script.python.PythonScriptEngineTests:
   >    1) Thread[id=12, name=org.python.google.common.base.internal.Finalizer, state=WAITING, group=TGRP-PythonScriptEngineTests]
   >         at java.lang.Object.wait(Native Method)
   >         at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:135)
   >         at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:151)
   >         at org.python.google.common.base.internal.Finalizer.run(Finalizer.java:127)
   >    at __randomizedtesting.SeedInfo.seed([7A5ECFD8D0474383]:0)
   > Throwable #2: com.carrotsearch.randomizedtesting.ThreadLeakError: There are still zombie threads that couldn't be terminated:
   >    1) Thread[id=12, name=org.python.google.common.base.internal.Finalizer, state=WAITING, group=TGRP-PythonScriptEngineTests]
   >         at java.lang.Object.wait(Native Method)
   >         at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:135)
   >         at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:151)
   >         at org.python.google.common.base.internal.Finalizer.run(Finalizer.java:127)
   >    at __randomizedtesting.SeedInfo.seed([7A5ECFD8D0474383]:0)
```

Closes #22.

pom.xml

@@ -66,7 +66,7 @@
         <dependency>
             <groupId>org.python</groupId>
             <artifactId>jython-standalone</artifactId>
-            <version>2.5.3</version>
+            <version>2.7-b3</version>
             <scope>compile</scope>
         </dependency>