cache gx directory on Travis

.travis.yml

@@ -25,3 +25,7 @@ after_success:
   - cat go-libp2p-quic-transport.coverprofile > coverage.txt
   - cat */*.coverprofile >> coverage.txt
   - bash <(curl -s https://codecov.io/bash) -f coverage.txt
+
+cache:
+  directories:
+    - $GOPATH/src/gx