Bug 1457863 - 1. Update kotlin to 1.2.41; r=nalexander

Update the kotlin version used for GeckoView tests to 1.2.41. Also turn
on "warnings-as-errors", which is a compiler option introduced in kotlin
1.2.

MozReview-Commit-ID: 7cL2v4jFJ27

--HG--
extra : rebase_source : daf3c2f3ca24304faa10c399c4ef2bd145395ada

build.gradle

@@ -50,7 +50,7 @@ buildscript {
         }
     }
 
-    ext.kotlin_version = '1.1.51'
+    ext.kotlin_version = '1.2.41'
     ext.support_library_version = '23.4.0'
 
     if (gradle.mozconfig.substs.MOZ_ANDROID_GOOGLE_PLAY_SERVICES) {

mobile/android/geckoview/build.gradle

@@ -174,6 +174,10 @@ tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile) {
         }
     } as StandardOutputListener
 
+    kotlinOptions {
+        allWarningsAsErrors = true
+    }
+
     doFirst {
         logging.addStandardErrorListener(listener)
     }