hubot/sgi.js: Add Robot.react to Hubot >= v3.0.0

Addresses the issue described in #15. In short, hubot-test-helper v1.8.0
depends on hubot-v3.0.0, both are rewrites from CoffeeScript to ES2015,
and consequently hubot-slack adds Robot.react to a CoffeeScript-
compatible wrapper function, leading to smoke and integration test
failures.

The implementation of slackapi/hubot-slack#429 (CoffeeScript-to-ES2015
rewrite) should include a fix that will eliminate the need for this
workaround.

hubot/slack-github-issues.js

@@ -11,6 +11,12 @@
 var path = require('path');
 var slackGitHubIssues = require('..');
 
+// Remove this block and close #15 when slackapi/hubot-slack#429 is done.
+require('hubot-slack');
+var CoffeeScriptCompatibleHubot = require('hubot');
+var Hubot = require('hubot/es2015');
+Hubot.Robot.prototype.react = CoffeeScriptCompatibleHubot.Robot.prototype.react;
+
 function configParams() {
   var params = {updates: {}};