Riak: Drop old cookbook and install from apt repo

Close travis-ci/travis-ci#1090

I renounced to use a more recent version of Basho cookbook (e.g. 2.4.0)
because it relies on different cookbook dependencies for Erlang and
apt repository was recommeded by Sean Cribbs himself, as illustrated in
projects like https://github.com/basho/riak-python-client
Riak: Use backward-compatible configuration

For now, it might be safer to keep Riak configured as usual:
These app.config and vm.args correspond former/tweaked Riak cookbook
restult.

See also:
- 5f40180cbc4d9628121b0d8d6755ee654e87503a
- bcdc0bbc17b6d899d88cce04a390108a1fdfad2a

Notes:
- app.config and vm.args are intentionally organized as 'template'
  resources (instead of 'file'), having in mind some upcoming
  customization evolutions.
- 32-bit packaging is no more required since Travis VMs are now
  64-bit
Merge pull request #240 from gildegoma/riak-from-apt

Install Riak (1.4.2-1) from official Basho APT repository