Add cache entries to .travis.yml.

This is recommended by the Travis docs to speed up the parts of the
tests where unpack / install has a significant cost, as opposed to the
download.

This changeset adds entries for ~/.m2 (for Maven) and
~/Library/Caches/Homebrew for (brew on macOS).

.travis.yml

@@ -2,6 +2,11 @@ sudo: true
 
 language: java
 
+cache:
+  directories:
+  - $HOME/.m2
+  - $HOME/Library/Caches/Homebrew
+
 matrix:
   include:
     - os: linux