bump scala to 2.12.7 (#329)

diff --git a/.travis.yml b/.travis.yml
index ee628f4..ea19dee 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -7,7 +7,7 @@
 
 language: scala
 scala:
-   - 2.11.8
+   - 2.12.7
 
 services:
   - docker