fix travis-ci cache directories setting

Author: xuwei-k

.travis.yml

@@ -14,7 +14,7 @@ sudo: false
 cache:
   directories:
     - $HOME/.ivy2/cache
-    - $HOME/.sbt/boot/scala-$TRAVIS_SCALA_VERSION
+    - $HOME/.sbt/launchers
 
 scala:
   - 2.12.8