Remove implicit request dependency

[draaglom]

Motivation: request is implicitly depended on by
botbuilder-adapter-slack but it's not in package.json.

This means we want to either:

• Add it as an explicit dependency
• Remove it entirely

Given request is long since deprecated and usage in
botbuilder-adapter-slack is minimal (just this one request), the
latter seems most sensible.

Of candidate HTTP clients to swap out, node-fetch seems the obvious choice:

• It's very small
• It's popular
• some of the botbuilder/botkit packages depend on it already

Where I'd like your advice with this PR:

• This is technically a breaking change for anyone who is depending on getting errors specifically from Request
• I've bumped to a v2 package-lock in the process, hope you don't mind
• I've bumped down the dependency from botbuilder-adapter-slack to botkit to version 4.10 since npm couldn't find a 4.15, but this could just be my unfamiliarity with how lerna works (?)

[ghost]

All CLA requirements met.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.