Fix Mattermost adapter

[blag]

This is the fix for the Mattermost adapter from this forum post.

I have tested this manually with a minimal Mattermost server in a VM.

The slack-attachment event was removed from the hubot-matteruser dependency in PR 71, but our Mattermost adapter did not get updated to avoid using that.

I switched over to using the send method from hubot-matteruser.

Note: That methods accepts two parameters: envelope and a variadic parameter strings, but contrary to the name, the strings parameter also works just fine if each "string" is a message-like object, because it calls postMessage from mattermost-client, and that handles objects:

    postMessage(msg, channelID) {
        const postData = {
            message: msg,
            file_ids: [],
            create_at: 0,
            user_id: this.self.id,
            channel_id: channelID
        };

        if (typeof msg === 'string') {
          postData.message = msg;
        } else {
          postData.message = msg.message;
          if (msg.props) {
            postData.props = msg.props;
          }
          if (msg.file_ids) {
            postData.file_ids = msg.file_ids;
          }
        }

^{(reformatted that code slightly to improve readability)}

Once #190 and #191 are merged, I will either rebase and merge this with GitHub, or I will manually rebase this branch on #191, force push, and merge it.

[arm4b]

We'll need a Changelog for this PR.
Based on #193 release, Changelog sync-up in master should eventually happen.

[blag]

@armab I added a change to the changelog. Since I won't ever be merging the changes from #193 directly into master, the changelogs will differ, but this and #193 both fix the same thing. Let me know if this is acceptable to you.

[arm4b]

As mentioned in #193, master should have in its Changelog record about every release that happened in the project. It's not OK to compose master Changelog like 0.9.7 release never happened.

So let's please mention in Changelog a few notes about 0.9.7 for our users to have this information visible in history. This is not any worse then split-brain with patch and I hope its first and last time we're doing this.

Additionally, are you going to release v0.10.0? Please prepare the package version and changelog for that as well in PR.

[blag]

I closed #193. We won't do a split brain release.

I have a version bump local branch ready to go, but I'd like to merge this and then rebase that branch on top of master before I push it and open a PR. This is so I don't have to do a rebase and force push (since I know you don't like those).

[arm4b]

Sure, works for me.