Add Hugo CI Step to Validate Viper

Since Hugo is such a heavy user of Viper, this patch adds an
after_success section to the Travis-CI build that validates the Viper
commit by attempting to build Hugo and executing `hugo -s docs`.

This patch handles issue #222.

.travis.yml

@@ -1,3 +1,5 @@
+go_import_path: github.com/spf13/viper
+
 language: go
 go:
   - 1.5.4
@@ -15,5 +17,11 @@ matrix:
   fast_finish: true
 
 script:
+  - go install ./...
   - go test -v ./...
+
+after_success:
+  - go get -u -d github.com/spf13/hugo
+  - cd $GOPATH/src/github.com/spf13/hugo && make && ./hugo -s docs && cd -
+
 sudo: false