Remove "go mod download" and replace 1.9.x with 1.11.x due to missing go modules support

.travis.yml

@@ -3,7 +3,7 @@ dist: xenial
 language: go
 
 go:
-- 1.9.x
+- 1.11.x
 - 1.13.x
 
 go_import_path: "github.com/pterodactyl/wings"
@@ -19,8 +19,6 @@ install:
 - go get github.com/haya14busa/goverage
 - go get github.com/schrej/godacov
 
-- go mod download
-
 script:
 - make cross-build
 - goverage -v -coverprofile=coverage.out ./...