Upgrade to Kotlin 1.1.51

The main new feature available in this new version is null-safety of
Spring APIs (Framework, Data, Reactor) with Boot 2 without requiring
jsr305.jar to be present in project classpath.

-Xjsr305=strict compiler flag is not enabled by default because it is
considered as experimental, see
https://docs.spring.io/spring/docs/current/spring-framework-reference/kotlin.html#null-safety
for more details.

Closes gh-515

initializr-service/src/main/resources/application.yml

@@ -93,7 +93,7 @@ initializr:
     gradle:
       dependency-management-plugin-version: 0.6.0.RELEASE
     kotlin:
-      version: 1.1.4-3
+      version: 1.1.51
   dependencies:
     - name: Core
       content: