Renaming index.src.html to index.bs

The Bikeshed documentation indicates that the preferred filename for the
input file into Bikeshed tool should use the suffix ".bs" [1]. We chose to
start with ".src.html" instead because the templates we were working from
used that form, but it's now deprecated.

Does anyone object to me making a commit to update to a ".bs" extension
(using the `git mv` command, of course)? There will also be some changes to
the Makefile for the TravisCI automation.

If I don't hear objections in the next day or so, I'll make the edit.

Cheers,
J.C.

1) https://github.com/tabatkins/bikeshed#bikeshed-file-extension