updated readme with latest plugin version

To avoid getting an error on sbt, about not finding 

```
[info] Resolving org.scala-sbt#precompiled-2_10_0-m7;0.12.1 ...
[warn] 	::::::::::::::::::::::::::::::::::::::::::::::
[warn] 	::          UNRESOLVED DEPENDENCIES         ::
[warn] 	::::::::::::::::::::::::::::::::::::::::::::::
[warn] 	:: org.jboss.shrinkwrap.descriptors#shrinkwrap-descriptors-api-base;1.0.3.Final: not found
[warn] 	:: org.jboss.shrinkwrap.descriptors#shrinkwrap-descriptors-spi;1.0.3.Final: not found
```