Use standard require path for the base module

I guess the reason for absolute file system paths have been weird class
loading problems caused by Knife::SubcommandLoader. IMHO the right way
to handle those is to fix Knife[1], and in the meanwhile inform users to
run `gem cleanup knife-digital_ocean` to get rid of the old gem
versions.

[1]: http://tickets.opscode.com/browse/CHEF-3255
Remove obsolete dependencies

Readline and Chef::JSONCompat are not used anywhere.
And we only need 'socket', not 'net/ssh/multi'.
Make it possible for DigitalOceanBase's includers to use Knife.deps

Later `Knife.deps` calls override earlier blocks[1], so we have to
create our own way of chaining lazy dependency loaders.

Add `DigitalOceanBase.load_deps` and inject an automatic call to it in
the inheriting class (that works only if the said class does not need
to declare `deps` itself).

[1]: http://tickets.opscode.com/browse/CHEF-3932
Move `droplet create` deps to correct class