Consider pointing binder icon to latest release branch

[jarmarshall]

Currently the binder icon in README.md and getting_started.rst point to the master branch. It may be better to have these point to the most recent release branch instead. If this is agreed then the following actions need to be taken:

• add a list of previous binder references, possibly under mybinder.org in about.rst
• update development.rst to include instructions on bumping the binder branch reference in README.md and getting_started.rst, and adding the previous binder branch reference to the list of previous releases, to enable ongoing access

[willfurnass]

Yes, could have separate mybinder buttons/links for the latest release and for the HEAD of master.

[willfurnass]

mybinder.org links in README and docs will now start a Jupyter session for MuMoT v1.1.1 instead of HEAD of master (PR #369).

Still need to address the points above.