Add installation instructions for 1.3.1 packages

This addresses the issue #411 and basho/riak#332 where the links
were missing because 1.3.0 was no longer the 'latest' version
in the docs.
Address review comments
Add missing 'wget' commands
Merge pull request #412 from basho/jem-ubuntu-pkg-install

Add installation instructions for 1.3.1 packages