Bump github.com/gorilla/mux from 1.8.0 to 1.8.1

Bumps [github.com/gorilla/mux](https://github.com/gorilla/mux) from 1.8.0 to 1.8.1.
- [Release notes](https://github.com/gorilla/mux/releases)
- [Commits](gorilla/mux@v1.8.0...v1.8.1)

---
updated-dependencies:
- dependency-name: github.com/gorilla/mux
  dependency-version: 1.8.1
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

go.mod

@@ -13,7 +13,7 @@ require (
 	github.com/dnaeon/go-vcr v1.0.1 // indirect
 	github.com/fcjr/aia-transport-go v1.2.1
 	github.com/gomodule/redigo v2.0.0+incompatible
-	github.com/gorilla/mux v1.8.0
+	github.com/gorilla/mux v1.8.1
 	github.com/gregjones/httpcache v0.0.0-20190611155906-901d90724c79
 	github.com/jamiealquiza/envy v1.1.0
 	github.com/muesli/smartcrop v0.3.0