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

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

@@ -0,0 +1,59 @@
+### v0.6.0 - August 31, 2019
+full differences can be viewed using `git log --oneline --decorate --color v0.5.0..v0.6.0`
+thanks to everyone who has contributed since January!
+
+
+#### Breaking Changes:
+- Info struct has had fields removed related to deprecated functionality by slack.
+- minor adjustments to some structs.
+- some internal default values have changed, usually to be more inline with slack defaults or to correct inability to set a particular value. (Message Parse for example.)
+
+##### Highlights:
+- new slacktest package easy mocking for slack client. use, enjoy, please submit PRs for improvements and default behaviours! shamelessly taken from the [slack-test repo](https://github.com/lusis/slack-test) thank you lusis for letting us use it and bring it into the slack repo.
+- blocks, blocks, blocks.
+- RTM ManagedConnection has undergone a significant cleanup.
+in particular handles backoffs gracefully, removed many deadlocks,
+and Disconnect is now much more responsive.
+
+### v0.5.0 - January 20, 2019
+full differences can be viewed using `git log --oneline --decorate --color v0.4.0..v0.5.0`
+- Breaking changes: various old struct fields have been removed or updated to match slack's api.
+- deadlock fix in RTM disconnect.
+
+### v0.4.0 - October 06, 2018
+full differences can be viewed using `git log --oneline --decorate --color v0.3.0..v0.4.0`
+- Breaking Change: renamed ApplyMessageOption, to mark it as unsafe,
+this means it may break without warning in the future.
+- Breaking: Msg structure files field changed to an array.
+- General: implementation for new security headers.
+- RTM: deadlock fix between connect/disconnect.
+- Events: various new fields added.
+- Web: various fixes, new fields exposed, new methods added.
+- Interactions: minor additions expect breaking changes in next release for dialogs/button clicks.
+- Utils: new methods added.
+
+### v0.3.0 - July 30, 2018
+full differences can be viewed using `git log --oneline --decorate --color v0.2.0..v0.3.0`
+- slack events initial support added. (still considered experimental and undergoing changes, stability not promised)
+- vendored depedencies using dep, ensure using up to date tooling before filing issues.
+- RTM has improved its ability to identify dead connections and reconnect automatically (worth calling out in case it has unintended side effects).
+- bug fixes (various timestamp handling, error handling, RTM locking, etc).
+
+### v0.2.0 - Feb 10, 2018
+
+Release adds a bunch of functionality and improvements, mainly to give people a recent version to vendor against.
+
+Please check [0.2.0](https://github.com/nlopes/slack/releases/tag/v0.2.0)
+
+### v0.1.0 - May 28, 2017
+
+This is released before adding context support.
+As the used context package is the one from Go 1.7 this will be the last
+compatible with Go < 1.7.
+
+Please check [0.1.0](https://github.com/nlopes/slack/releases/tag/v0.1.0)
+
+### v0.0.1 - Jul 26, 2015
+
+If you just updated from master and it broke your implementation, please
+check [0.0.1](https://github.com/nlopes/slack/releases/tag/v0.0.1)