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

setup.py

@@ -44,7 +44,7 @@ setup(
 
     packages=find_packages(),
     entry_points={'console_scripts': ['yamllint=yamllint.cli:run']},
-    package_data={'yamllint': ['conf/*.yml'],
+    package_data={'yamllint': ['conf/*.yaml'],
                   'tests': ['yaml-1.2-spec-examples/*']},
     install_requires=['pyyaml'],
     tests_require=['nose'],