Add vagrant-digitalocean to the 'plugins' group and update the tag from which vagrant is used

This change allows you to `bundle exec vagrant --provider=digital_ocean`. Without the plugin being
in the 'plugins' group, it's difficult to test the plugin directly from the repo. See
https://docs.vagrantup.com/v2/plugins/packaging.html for more information.
Modify to use the v2 API
Merge pull request #125 from ijin/api-v2

v2.0 API support
Update setup_key.rb

Public key field was updated according to https://developers.digitalocean.com/#keys
Merge pull request #126 from agilestack/master

Using right field name for public ssh key in API v2.0
Merge pull request #119 from skottler/gem_group

Add vagrant-digitalocean to the 'plugins' group and update the tag from ...
Dump the version to 0.6.0
v2.0 API pagination support
retrieve public ip_address correctly
single quote for type & public string
retrieve correct network address in create action
fix v2 api typo on auto backup feature when create droplet
Merge pull request #127 from kun432/api_v2_pagenation

v2.0 API pagination support
Get ready to release 0.6.1
Merge pull request #131 from simoniong/fix_typo_on_backups_feature

fix v2 api typo on auto backup feature when create droplet
Merge pull request #130 from simoniong/fix_private_network_bug

retrieve public ip_address correctly
Fixed non-working link to "Create droplet" form
Merge pull request #140 from khromov/patch-1

Fixed non-working link to "Create droplet" form
Merge remote-tracking branch 'upstream/master'