Update plugins.sbt : correct version of sbt-plugin

Fixing this issue:

```
[warn] 	Note: Some unresolved dependencies have extra attributes.  Check that these dependencies exist with the requested attributes.
[warn] 		com.typesafe.play:sbt-plugin:2.0 (sbtVersion=0.13, scalaVersion=2.10)
[warn] 
sbt.ResolveException: unresolved dependency: com.typesafe.play#sbt-plugin;2.0: not found
	at sbt.IvyActions$.sbt$IvyActions$$resolve(IvyActions.scala:213)

```