Issue-268: Websocket login respects https_proxy environment variable

[liuweichu]

• [Y] I've read and understood the Contributing guidelines and have done my best effort to follow them.
• [Y] I've read and agree to the Code of Conduct.
• [Y] I've been mindful about doing atomic commits, adding documentation to my changes, not refactoring too much.
• [Y] I've a descriptive title and added any useful information for the reviewer. Where appropriate, I've attached a screenshot and/or screencast (gif preferrably).
• I've written tests to cover the new code and functionality included in this PR.
• [Y] I've read, agree to, and signed the Contributor License Agreement (CLA).

PR Summary

If https_proxy is set, slack websocket will use the proxy to connect to slack server.
This solves the issue that hubot-slack cannot login behind firewall.

Related Issues

Fixes #268

Test strategy

The code is running great in our production env, no test case is written though. (If test is required, please guide me how to write

[UncannyBingo]

@aoberoi Could I convince you give this a review? I'm inclined to 👍 but I'd like your eyes on it too.

[UncannyBingo]

In the meantime, @liuweichu I see that the linter is rejecting your changes. In particular, we require string literals to use single-quotes, and all lines to be terminated with semi-colons. These issues must be resolved before we can accept this PR.

I apologize that this has taken me so long to come back around to, and I am eager to get this merged in!

[UncannyBingo]

Unfortunately, it doesn't look like @liuweichu actually signed the CLA. We cannot approve without a signed CLA :(

[liuweichu]

@DEGoodmanWilson Thanks for lintering

• Rebased on current master
• " -> '
• ; added

[coveralls]

Coverage decreased (-0.06%) to 85.603% when pulling e0ccfc7 on liuweichu:master into ce618b1 on slackapi:master.

[coveralls]

Coverage decreased (-0.06%) to 85.603% when pulling e0ccfc7 on liuweichu:master into ce618b1 on slackapi:master.

[coveralls]

Coverage decreased (-0.06%) to 85.603% when pulling 4181511 on liuweichu:master into fdd3717 on slackapi:master.

[aoberoi]

Oops I missed this mention! I'll find a better way to deal with the notifications moving forward.