fix travis cache directories

sbt/website#115

.travis.yml

@@ -5,18 +5,6 @@ cache:
   directories:
     - $HOME/.ivy2/cache
 
-    # At the moment, sbt 0.13.5 is preinstalled in Travis VM image,
-    # which fortunately corresponds to current scalaz settings.
-    # The line below can be used to cache a given sbt version.
-    # - $HOME/.sbt/launchers/0.13.x
-
-    # At the moment, Scala 2.11.2 and 2.10.4 are preinstalled in Travis VM
-    # image, which fortunately corresponds to current scalaz settings.
-    # The line below is used to cache the scala version used by the build
-    # job, as these versions might be replaced after a Travis CI build
-    # environment upgrade (e.g. scala 2.11.2 could be replaced by scala 2.11.4).
-    - $HOME/.sbt/boot/scala-$TRAVIS_SCALA_VERSION
-
 language: scala
 scala:
   - 2.12.1