Add cache configuration to .travis.yml

This should speed up builds a bit

License: MIT
Signed-off-by: Hector Sanjuan <code@hector.link>

.travis.yml

@@ -35,3 +35,6 @@ deploy:
   script: docker run -v $(pwd):$(pwd) -t snapcore/snapcraft sh -c "apt update -qq
     && cd $(pwd) && snapcraft && snapcraft push *.snap --release edge"
   skip_cleanup: true
+cache:
+  directories:
+    - $GOPATH/src/gx