Removed the vendored db.
Removed code related to the vendored ruby-advisory-db.

* Added Database.path= to configure the default db. This will also be
  used in the specs.
* Added Database#update! as well.
Update the database before every scan.
No longer add data/ruby-advisory-db paths to the gemspec.files.
Don't clone the entire ruby-advisory-db history.
Added Database#last_updated to determine how old the db is.
Updated the Database specs.
Just require bundler/setup.
Removed reference to old constant.
Added Database.download!
Renamed Database.download! to install!
Merge remote-tracking branch 'upstream/gut_vendored_db'

Conflicts:
	Rakefile
	data/ruby-advisory-db
	data/ruby-advisory-db.ts
	spec/spec_helper.rb
Whitespace.
Make advisory_spec pass.

It can now be run standalone as well, and doesn't require the overhead of
cloning the advisory repo.
Fix bad method name.
Fix integration_spec, and clarify semantics.