Bump github.com/spf13/viper from 1.8.1 to 1.15.0

Bumps [github.com/spf13/viper](https://github.com/spf13/viper) from 1.8.1 to 1.15.0.
- [Release notes](https://github.com/spf13/viper/releases)
- [Commits](spf13/viper@v1.8.1...v1.15.0)

---
updated-dependencies:
- dependency-name: github.com/spf13/viper
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

go.mod

@@ -4,7 +4,8 @@ go 1.12
 
 require (
 	github.com/mitchellh/go-homedir v1.1.0
+	github.com/pelletier/go-toml v1.9.5 // indirect
 	github.com/spf13/cobra v0.0.7
-	github.com/spf13/viper v1.14.0
+	github.com/spf13/viper v1.15.0
 	sigs.k8s.io/yaml v1.2.0
 )