Add vagrant-digitalocean to the list of supported providers

We even have acceptance tests for it.
Lock vagrant-rackspace down to v0.1.2

Version 0.1.3 has a conflicting fog dependency with vagrant-aws v0.3.0.
Don't install provider plugins in Travis

They are only needed on acceptance tests.
Compatibility with vagrant-aws v0.4.0

Next vagrant-aws release will remove its custom TimedProvision class and
use only the standard Builtin::Provision instead.

To support both old and new versions of the plugin, check if the
TimedProvision class is defined before using it.
Merge pull request #43 from tmatilai/travis

Fix development dependencies and Travis tests
Merge pull request #41 from tmatilai/readme-do

Add vagrant-digitalocean to the list of supported providers
Merge pull request #45 from tmatilai/aws-0.4

Compatibility with vagrant-aws v0.4.0