Distribution: Restore spec examples in package_data

Put `tests/yaml-1.2-spec-examples/*` back in `setup.py`'s `package_data`
because they need to be installed when running `python setup.py build`,
so Debian packaging script `dh_auto_test -O--buildsystem=pybuild`
doesn't fail.

See also commit e6dc67f.

MANIFEST.in

@@ -1,4 +1,3 @@
 include LICENSE
 include README.rst
 include docs/*
-include tests/yaml-1.2-spec-examples/*

setup.py

@@ -44,7 +44,8 @@ setup(
 
     packages=find_packages(),
     entry_points={'console_scripts': ['yamllint=yamllint.cli:run']},
-    package_data={'yamllint': ['conf/*.yml']},
+    package_data={'yamllint': ['conf/*.yml'],
+                  'tests': ['yaml-1.2-spec-examples/*']},
     install_requires=['pyyaml'],
     tests_require=['nose'],
     test_suite='nose.collector',