Cache go directories on Travis

DirectXMan12 · DirectXMan12 · commit 81c9bf574ea8 · 2019-05-15T13:43:22.000-07:00
This caches the go mod and go build cache directories, which should
speed up test runs.
diff --git a/.travis.yml b/.travis.yml
@@ -4,6 +4,11 @@ os:
   - linux
   - osx
 
+cache:
+  directories:
+  - $HOME/.cache/go-build
+  - $GOPATH/pkg/mod
+
 go:
 - "1.12"
 
