Remove replace directives and use own fork to make go get work again (#1028)

See https://github.com/golang/go/issues/30354
go get doesn't honor the go.mod replace options.

vendor/github.com/bwmarrin/discordgo/.travis.yml

@@ -1,14 +0,0 @@
-language: go
-go:
-    - 1.9.x
-    - 1.10.x
-    - 1.11.x
-install:
-    - go get github.com/bwmarrin/discordgo
-    - go get -v .
-    - go get -v golang.org/x/lint/golint
-script:
-    - diff <(gofmt -d .) <(echo -n)
-    - go vet -x ./...
-    - golint -set_exit_status ./...
-    - go test -v -race ./...