update gradle-api artifact to version 1.6 this is required to make kotlin gradle plugin compatible with upcoming Gradle 2.0
cleanup Gradle API deprecation warnings to make plugin compatible with upcoming Gradle 2.0. This includes moving minimum compatible version from 1.4 to 1.6
Merge remote-tracking branch 'origin/pr/465'

Conflicts:
	libraries/tools/kotlin-gradle-plugin-core/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/KotlinPlugin.kt