Simplify sbt cache in Travis

https://www.scala-sbt.org/1.0/docs/Travis-CI-with-sbt.html

Author: ashawley

.travis.yml

@@ -38,5 +38,4 @@ before_cache:
 cache:
   directories:
   - $HOME/.ivy2/cache
-  - $HOME/.sbt/boot
-  - $HOME/.sbt/launchers
+  - $HOME/.sbt