exclude specs2 from lift-json 2.5-M4

https://github.com/lift/framework/issues/1397
https://github.com/lift/framework/blob/2.5-M4-release/project/Dependencies.scala#L86
http://repo1.maven.org/maven2/net/liftweb/lift-json_2.10/2.5-M4/lift-json_2.10-2.5-M4.pom