Bump github.com/gorilla/mux from 1.7.4 to 1.8.1

Bumps [github.com/gorilla/mux](https://github.com/gorilla/mux) from 1.7.4 to 1.8.1.
- [Release notes](https://github.com/gorilla/mux/releases)
- [Commits](gorilla/mux@v1.7.4...v1.8.1)

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

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

go.mod

@@ -4,7 +4,7 @@ go 1.12
 
 require (
 	github.com/golang/freetype v0.0.0-20170609003504-e2365dfdc4a0
-	github.com/gorilla/mux v1.7.4
+	github.com/gorilla/mux v1.8.1
 	github.com/kballard/go-shellquote v0.0.0-20180428030007-95032a82bc51
 	github.com/mattermost/mattermost-server/v5 v5.24.0-rc1
 	github.com/mholt/archiver/v3 v3.3.0

go.sum

@@ -189,8 +189,9 @@ github.com/gopherjs/gopherjs v0.0.0-20190910122728-9d188e94fb99/go.mod h1:wJfORR
 github.com/gorilla/context v1.1.1/go.mod h1:kBGZzfjB9CEq2AlWe17Uuf7NDRt0dE0s8S51q0aT7Yg=
 github.com/gorilla/handlers v1.4.2/go.mod h1:Qkdc/uu4tH4g6mTK6auzZ766c4CA0Ng8+o/OAirnOIQ=
 github.com/gorilla/mux v1.6.2/go.mod h1:1lud6UwP+6orDFRuTfBEV8e9/aOM/c4fVVCaMa2zaAs=
-github.com/gorilla/mux v1.7.4 h1:VuZ8uybHlWmqV03+zRzdwKL4tUnIp1MAQtp1mIFE1bc=
 github.com/gorilla/mux v1.7.4/go.mod h1:DVbg23sWSpFRCP0SfiEN6jmj59UnW/n46BH5rLB71So=
+github.com/gorilla/mux v1.8.1 h1:TuBL49tXwgrFYWhqrNgrUNEY92u81SPhu7sTdzQEiWY=
+github.com/gorilla/mux v1.8.1/go.mod h1:AKf9I4AEqPTmMytcMc0KkNouC66V3BtZ4qD5fmWSiMQ=
 github.com/gorilla/schema v1.1.0/go.mod h1:kgLaKoK1FELgZqMAVxx/5cbj0kT+57qxUrAlIO2eleU=
 github.com/gorilla/websocket v1.4.0/go.mod h1:E7qHFY5m1UJ88s3WnNqhKjPHQ0heANvMoAMk2YaljkQ=
 github.com/gorilla/websocket v1.4.2 h1:+/TMaTYc4QFitKJxsQ7Yye35DkWvkdLcvGKqM+x0Ufc=