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

vendor/github.com/slack-go/slack/README.md

@@ -0,0 +1,96 @@
+Slack API in Go [![GoDoc](https://godoc.org/github.com/slack-go/slack?status.svg)](https://godoc.org/github.com/slack-go/slack) [![Build Status](https://travis-ci.org/slack-go/slack.svg)](https://travis-ci.org/slack-go/slack)
+===============
+This is the original Slack library for Go created by Norberto Lopez, transferred to a Github organization.
+
+[![Join the chat at https://gitter.im/go-slack/Lobby](https://badges.gitter.im/go-slack/Lobby.svg)](https://gitter.im/go-slack/Lobby?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+
+This library supports most if not all of the `api.slack.com` REST
+calls, as well as the Real-Time Messaging protocol over websocket, in
+a fully managed way.
+
+
+
+
+## Changelog
+
+[CHANGELOG.md](https://github.com/slack-go/slack/blob/master/CHANGELOG.md) is available. Please visit it for updates.
+
+## Installing
+
+### *go get*
+
+    $ go get -u github.com/slack-go/slack
+
+## Example
+
+### Getting all groups
+
+```golang
+import (
+	"fmt"
+
+	"github.com/slack-go/slack"
+)
+
+func main() {
+	api := slack.New("YOUR_TOKEN_HERE")
+	// If you set debugging, it will log all requests to the console
+	// Useful when encountering issues
+	// slack.New("YOUR_TOKEN_HERE", slack.OptionDebug(true))
+	groups, err := api.GetGroups(false)
+	if err != nil {
+		fmt.Printf("%s\n", err)
+		return
+	}
+	for _, group := range groups {
+		fmt.Printf("ID: %s, Name: %s\n", group.ID, group.Name)
+	}
+}
+```
+
+### Getting User Information
+
+```golang
+import (
+    "fmt"
+
+    "github.com/slack-go/slack"
+)
+
+func main() {
+    api := slack.New("YOUR_TOKEN_HERE")
+    user, err := api.GetUserInfo("U023BECGF")
+    if err != nil {
+	    fmt.Printf("%s\n", err)
+	    return
+    }
+    fmt.Printf("ID: %s, Fullname: %s, Email: %s\n", user.ID, user.Profile.RealName, user.Profile.Email)
+}
+```
+
+## Minimal RTM usage:
+
+See https://github.com/slack-go/slack/blob/master/examples/websocket/websocket.go
+
+
+## Minimal EventsAPI usage:
+
+See https://github.com/slack-go/slack/blob/master/examples/eventsapi/events.go
+
+
+## Contributing
+
+You are more than welcome to contribute to this project.  Fork and
+make a Pull Request, or create an Issue if you see any problem.
+
+Before making any Pull Request please run the following:
+
+```
+make pr-prep
+```
+
+This will check/update code formatting, linting and then run all tests
+
+## License
+
+BSD 2 Clause license