Set up pre-merge test build checks for Pull Requests

[AmeliaBR]

I just created two different PRs fixing build errors introduced by recently merged changes. It would be very useful if we had automated test builds set up as pre-merge checks on GitHub.

If anyone wants to look into doing this, probably through TravisCI, that would be very helpful.

In the meantime, I'm going to suggest that other editors be extra careful about merging an substantive changes without first checking out the PR branch and running a test build locally.

[AmeliaBR]

In addition:

For changes to the build scripts themselves, the build system for svgwg.org needs to manually restarted (@heycam can do this). This is to prevent rogue scripts from causing mayhem on the server.

So we'd want some sort of extra warning showing up for these PRs. It wouldn't block merging, but it would remind the person doing the merging that an extra step is required.

[svgeesus]

Oops, it seems that we have lost Overview.html and changes.html from the built spec on svgwg.org

(Spotted because the changes link is now broken on our SVG2 CR transition request )