Emit token expiry event 1s earlier

This allows re-request st2 token in advance, instead of trying to get it when it's _already_ expired.
Add here HTTP request/response time to communicate token update and it leads to issues and races like StackStorm/st2#4119 StackStorm/hubot-stackstorm#178 (comment) StackStorm/hubot-stackstorm#157

index.js

@@ -189,7 +189,7 @@ module.exports = function (opts) {
           }
           this._expiryTimeout = setTimeout(function () {
             this.emit('expiry', token);
-          }.bind(this), new Date(token.expiry) - new Date());
+          }.bind(this), new Date(token.expiry) - new Date() - 1000);
           return token;
         }.bind(this));
       }