fix for goreleaser: install rpm to build rpm packages

Author: cha87de

.travis.yml

@@ -2,6 +2,10 @@ language: go
 go:
   - '1.9' 
 
+before_install:
+  - sudo apt-get -qq update
+  - sudo apt-get install -y rpm
+
 install:
   - "go get -d -v ./..."
   - "go get -u github.com/smartystreets/goconvey"