Use '.yaml' extension as default, not '.yml'

As someone said [1] on the internet:

    Say ".yaml" not ".yml".
    This is not MS-DOS, and YML is a Yahoo XML dialect.

Similarly, we use '.json', not '.jsn'.

[1]: https://github.com/ceph/s3-tests/commit/e17c56a

yamllint/config.py

@@ -129,7 +129,7 @@ def get_extended_config_file(name):
     # Is it a standard conf shipped with yamllint...
     if '/' not in name:
         std_conf = os.path.join(os.path.dirname(os.path.realpath(__file__)),
-                                'conf', name + '.yml')
+                                'conf', name + '.yaml')
 
         if os.path.isfile(std_conf):
             return std_conf