Use upstream slack-go/slack again (#1018)

vendor/github.com/slack-go/slack/.travis.yml

@@ -0,0 +1,39 @@
+language: go
+
+env:
+  - GO111MODULE=on
+
+install: true
+
+before_install:
+  - export PATH=$HOME/gopath/bin:$PATH
+  # install gometalinter
+  - curl -L https://git.io/vp6lP | sh
+
+script:
+  - PATH=$PWD/bin:$PATH gometalinter ./...
+  - go test -race -cover ./...
+
+matrix:
+  allow_failures:
+    - go: tip
+  include:
+  - go: "1.7.x"
+    script:  go test -v ./...
+  - go: "1.8.x"
+    script:  go test -v ./...
+  - go: "1.9.x"
+    script:  go test -v ./...
+  - go: "1.10.x"
+    script:  go test -v ./...
+  - go: "1.11.x"
+    script: go test -v -mod=vendor ./...
+  - go: "1.12.x"
+    script: go test -v -mod=vendor ./...
+  - go: "1.13.x"
+    script: go test -v -mod=vendor ./...
+  - go: "tip"
+    script: go test -v -mod=vendor ./...
+
+git:
+  depth: 10