Bump kotlinVersion from 1.3.50 to 1.3.60 (#279)

Bumps `kotlinVersion` from 1.3.50 to 1.3.60.

Updates `kotlin-gradle-plugin` from 1.3.50 to 1.3.60
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](https://github.com/JetBrains/kotlin/commits)

Updates `kotlin-stdlib` from 1.3.50 to 1.3.60
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](https://github.com/JetBrains/kotlin/commits)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

build.gradle

@@ -1,7 +1,7 @@
 buildscript {
     ext {
         javaVersion = '1.8'
-        kotlinVersion = '1.3.50'
+        kotlinVersion = '1.3.60'
 
         // see https://www.jetbrains.com/intellij-repository/releases/
         // and https://www.jetbrains.com/intellij-repository/snapshots/