Nesta::Config.can_use_yaml? is now redundant
Nesta::Config.can_use_yaml? is now redundant

See the previous commit from Glenn; the concept of when you can use Yaml
is now as simple as "does it exist?" It used to mean "is a NESTA_...
environment var defined, and if not, does it exist?".

nathandao/ginatra_dummy_2,gma/nesta,nathandao/ginatra_dummy_2,nathandao/ginatra_dummy_2