Add support for deploying arbitrary commit hash IDs

[bryanbraun]

Currently, this gem allows us to deploy a default branch, and an arbitrary branch, using the branch=branch-name option.

If we try to use this option to deploy a commit hash ID, it fails:

mina deploy on="staging" branch="97c3f0eb8746768a50ba807fd96f35a62a4804bc"
/Users/yosevukilonzo/.rvm/rubies/ruby-2.3.1/lib/ruby/2.3.0/YAML.rb:14: warning: already initialized constant YAML
/Users/yosevukilonzo/.rvm/rubies/ruby-2.3.1/lib/ruby/2.3.0/yaml.rb:14: warning: previous definition of YAML was here
       [mina-circle] Fetching: seesparkbox-dist.tar.gz
Sending URL: https://circleci.com/api/v1.1/project/github/sparkbox/seesparkbox.com/tree/97c3f0eb8746768a50ba807fd96f35a62a4804bc?filter=successful&has_artifacts=true&circle-token=00866f8f660a4d197cc03e7b270e04dddf31b463
mina aborted!
NoMethodError: undefined method `[]' for nil:NilClass
/Users/yosevukilonzo/.rvm/gems/ruby-2.3.1/gems/mina-circle-1.4.2/lib/mina-circle/circle-ci/project.rb:44:in `fetch_artifacts'
/Users/yosevukilonzo/.rvm/gems/ruby-2.3.1/gems/mina-circle-1.4.2/lib/mina-circle/circle-ci/project.rb:12:in `artifacts'
/Users/yosevukilonzo/.rvm/gems/ruby-2.3.1/gems/mina-circle-1.4.2/lib/mina-circle/circle-ci/circle_ci.rb:9:in `artifact_url'
...

The main issue is this:

NoMethodError: undefined method `[]' for nil:NilClass

Deploying arbitrary hash IDs is often supported in deploy tools like this. What do you think about adding that feature?