Add support to both .yml and .yaml

[rgaiacs]

Closes #1373

[minrk]

These repetitions make me think perhaps we want a self.environment_yaml attribute or property that's resolved once.

[rgaiacs]

We already have a self.environment_yaml attribute

repo2docker/repo2docker/buildpacks/conda/__init__.py

Lines 232 to 256 in ca21e1b

	_environment_yaml = None
	@property
	def environment_yaml(self):
	if self._environment_yaml is not None:
	return self._environment_yaml
	environment_yml = self.binder_path("environment.yml")
	if not os.path.exists(environment_yml):
	self._environment_yaml = {}
	return self._environment_yaml
	with open(environment_yml) as f:
	env = YAML().load(f)
	# check if the env file is empty, if so instantiate an empty dictionary.
	if env is None:
	env = {}
	# check if the env file provided a dict-like thing not a list or other data structure.
	if not isinstance(env, Mapping):
	raise TypeError(
	"environment.yml should contain a dictionary. Got %r" % type(env)
	)
	self._environment_yaml = env
	return self._environment_yaml

but it does something else.

[rgaiacs]

Refactoring required a fresh brain to accurately read the properties name. :-D

All tests are passing, including the small test added to check for this new feature.

This pull request is ready for review.

[minrk]

Looks great, thanks!