chore: testing rpmbuild not in path

and adding it to travis

.travis.yml

@@ -1,5 +1,8 @@
 dist: trusty
-sudo: required
+addons:
+  apt:
+    packages:
+    - rpmbuild
 language: go
 go: 1.9
 services:

rpm/rpm.go

@@ -55,7 +55,11 @@ func (*RPM) Package(info nfpm.Info, w io.Writer) error {
 	cmd.Dir = temps.Root
 	out, err := cmd.CombinedOutput()
 	if err != nil {
-		return errors.Wrapf(err, "rpmbuild failed: %s", string(out))
+		var msg = "rpmbuild failed"
+		if string(out) != "" {
+			msg += ": " + string(out)
+		}
+		return errors.Wrap(err, msg)
 	}
 
 	rpm, err := os.Open(temps.RPM)

rpm/rpm_test.go

@@ -2,6 +2,7 @@ package rpm
 
 import (
 	"io/ioutil"
+	"os"
 	"testing"
 
 	"github.com/goreleaser/nfpm"
@@ -54,10 +55,17 @@ func TestNoFiles(t *testing.T) {
 			Vendor:      "nope",
 			License:     "MIT",
 			Bindir:      "/usr/local/bin",
-			Files:       map[string]string{},
-			ConfigFiles: map[string]string{},
 		}),
 		ioutil.Discard,
 	)
 	assert.Error(t, err)
 }
+
+func TestRPMBuildNotInPath(t *testing.T) {
+	assert.NoError(t, os.Setenv("PATH", ""))
+	var err = Default.Package(
+		nfpm.WithDefaults(nfpm.Info{}),
+		ioutil.Discard,
+	)
+	assert.EqualError(t, err, `rpmbuild failed: exec: "rpmbuild": executable file not found in $PATH`)
+}