build(deps): bump github.com/spf13/viper from 1.7.1 to 1.8.1

Bumps [github.com/spf13/viper](https://github.com/spf13/viper) from 1.7.1 to 1.8.1.
- [Release notes](https://github.com/spf13/viper/releases)
- [Commits](spf13/viper@v1.7.1...v1.8.1)

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

Signed-off-by: dependabot[bot] <[email protected]>

go.mod

@@ -19,7 +19,7 @@ require (
 	github.com/go-kit/kit v0.11.0
 	github.com/gogo/gateway v1.1.0
 	github.com/gogo/protobuf v1.3.3
-	github.com/golang/mock v1.4.4
+	github.com/golang/mock v1.5.0
 	github.com/golang/protobuf v1.5.2
 	github.com/google/gofuzz v1.2.0
 	github.com/gorilla/handlers v1.5.1
@@ -39,18 +39,16 @@ require (
 	github.com/nxadm/tail v1.4.8 // indirect
 	github.com/onsi/ginkgo v1.15.0 // indirect
 	github.com/onsi/gomega v1.10.5 // indirect
-	github.com/pelletier/go-toml v1.8.0 // indirect
 	github.com/pkg/errors v0.9.1
 	github.com/prometheus/client_golang v1.11.0
 	github.com/prometheus/common v0.29.0
 	github.com/rakyll/statik v0.1.7
 	github.com/regen-network/cosmos-proto v0.3.1
 	github.com/rs/zerolog v1.23.0
-	github.com/spf13/afero v1.3.4 // indirect
 	github.com/spf13/cast v1.3.1
 	github.com/spf13/cobra v1.1.3
 	github.com/spf13/pflag v1.0.5
-	github.com/spf13/viper v1.7.1
+	github.com/spf13/viper v1.8.1
 	github.com/stretchr/testify v1.7.0
 	github.com/syndtr/goleveldb v1.0.1-0.20200815110645-5c35d600f0ca
 	github.com/tendermint/btcd v0.1.1