Gradle: set default values of languageVersion and apiVersion to null

Before the change, Gradle always explicitly set language and api
version to the latest version, preventing the compiler
from inferencing the arguments.

     #KT-18047 fixed

compiler/cli/cli-common/src/org/jetbrains/kotlin/cli/common/arguments/DefaultValues.kt

@@ -29,7 +29,7 @@ open class DefaultValues(val defaultValue: String, val possibleValues: List<Stri
     object StringNullDefault : DefaultValues("null")
 
     object LanguageVersions : DefaultValues(
-            "\"" + LanguageVersion.LATEST_STABLE.versionString + "\"",
+            "null",
             LanguageVersion.values().map { "\"${it.description}\"" }
     )