setup: define test dependencies with extras_require

tests_require is only supported by setuptools' test entry point which is
deprecated, extras_require is supported by most tools.

setup.py

@@ -29,10 +29,12 @@ setup(name=name,
           'setuptools',
           'slapos.recipe.build',
       ],
-      tests_require=[
-          'mock',
-          'pathlib',
-      ],
+      extras_require={
+          'test': [
+              'mock',
+              'pathlib',
+          ],
+      },
       entry_points={
           'zc.buildout': ['default = rubygems:Recipe']
       },

tox.ini

@@ -11,3 +11,4 @@ commands = python setup.py nosetests
 deps =
     coverage==3.7.1
     nose==1.3.4
+extras = test