Remove explicit Scala version for sbt project build files

Previously, `project/plugins.sbt` explicitly set scalaVersion to 2.10.4.  This
can cause issues when using a version of sbt that is compiled against a
different version of Scala (for example sbt 0.13.9 uses 2.10.5). Removing this
explict setting will cause build files to be compiled and run against the same
version of Scala that sbt is compiled against.

Note that this only applies to the project build files (items in project/), it
is distinct from the version of Scala we target for the actual spark
compilation.

Author: ahirreddy

project/plugins.sbt

@@ -1,5 +1,3 @@
-scalaVersion := "2.10.4"
-
 resolvers += Resolver.url("artifactory", url("http://scalasbt.artifactoryonline.com/scalasbt/sbt-plugin-releases"))(Resolver.ivyStylePatterns)
 
 resolvers += "Typesafe Repository" at "http://repo.typesafe.com/typesafe/releases/"