Travis CI: Enable caching for faster builds

.travis.yml

@@ -4,3 +4,12 @@ scala:
   - 2.11.5
 jdk:
   - oraclejdk8
+script:
+  - sbt ++2.11.5 test
+  # avoid unnecessary cache updates
+  - find $HOME/.sbt -name "*.lock" | xargs rm
+  - find $HOME/.ivy2 -name "ivydata-*.properties" | xargs rm
+cache:
+  directories:
+    - $HOME/.ivy2/cache
+    - $HOME/.sbt/boot/