fix master links to point to FA16 in README.md

[rjleveque]

The FA16 branch is the one for the published book and the README should point to files for this branch.

Also the binder link should start up a binder instance based on this branch.

I also updated the gh-pages branch (by pushing directly) so that the files

• http://www.clawpack.org/riemann_book/index.html
• http://www.clawpack.org/riemann_book/errata.html

better describe the branches, and the index now points to the FA16 branch rather than master.