.travis.yml: Dogfood git-validation

This makes it easier to test the script and avoids leaking dangling
whitespace, etc. into the repo.

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

.travis.yml

@@ -23,4 +23,5 @@ script:
   - go vet -x ./...
   - golint ./...
   - go build .
-  - go test -v ./...
+  - go test -v ./...
+  - ./git-validation -run DCO,short-subject,dangling-whitespace -v