update package jessevdk/go-flags to travis.yaml

Author: dixudx

.travis.yml

@@ -6,6 +6,7 @@ go:
 
 install:
   - if ! go get code.google.com/p/go.tools/cmd/cover; then go get golang.org/x/tools/cmd/cover; fi
+  - go get github.com/jessevdk/go-flags
 
 script:
   - go get