Install goimports for CI (#141)

Signed-off-by: wuxu <[email protected]>

Author: wuxuer

.travis.yml

@@ -10,6 +10,8 @@ go:
 env:
   - GO111MODULE=on
 
+before_install: go get golang.org/x/tools/cmd/goimports
+
 script:
   - diff -u <(echo -n) <(gofmt -d -s .)
   - diff -u <(echo -n) <(goimports -d .)