Switch to Scala 2.12.7 (#54)

diff --git a/settings.gradle b/settings.gradle
index bdd15bb..01343e2 100644
--- a/settings.gradle
+++ b/settings.gradle
@@ -27,7 +27,7 @@
 ]
 
 gradle.ext.scala = [
-    version: '2.11.8',
+    version: '2.12.7',
     compileFlags: ['-feature', '-unchecked', '-deprecation', '-Xfatal-warnings', '-Ywarn-unused-import']
 ]