Changes to Vagrant provisioner and environment to accomodate new
containerisation architecture.

Major changes include:

* Provisioning is done entirely with `vagrant up`. This has not been
easy and feels inelegant. Vagrant modifies the GEM_PATH env variable
meaning that it has to be saved and injected into subsequent `knife`
commands.

* Vagrantfile.local's symlinking magic is no longer needed

* DB reset now uses JSON fixtures rather than SQL.

* New ./dshell script for conveniently getting a terminal in a
container.

* There is an accompanying change in the cookbook which causes the
docker daemon service to delay its init until after the upstart
'vagrant-mounted' event has been triggered.
Updated readme to state correct username and pass created by DB reset
script.
USe new name for Vagrant Box to force downloading new box. Other bug
fixes.
Make code that gets user's default shell OSX compatible.
Use much simpler ENV['SHELL'] to get user's shell in Vagrantfile
Added script to install dev dependencies in deis-server. Used to run tests.
Install requirements.txt and dev_requirements.txt for Travis CI
Don't save user's original GEM env, just delete vagrant's GEM env from the ruby ENV var