Fix an issue in neighbors plugin which was introduced after subcategory support

It is not necessary that articles will have subcategories. You may
end up getting following critical error.

>
CRITICAL: ("'ArticlesGenerator' object has no attribute 'subcategories'",)
CRITICAL: 'ArticlesGenerator' object has no attribute 'subcategories'
Traceback (most recent call last):
  File "/Users/talha/Repos/VirtualEnvs/pelican-dev/bin/pelican", line 8, in <module>
    load_entry_point('pelican==3.3', 'console_scripts', 'pelican')()
  File "/Users/talha/Repos/VirtualEnvs/pelican-dev/lib/python2.7/site-packages/pelican-3.3-py2.7.egg/pelican/__init__.py", line 346, in main
    pelican.run()
  File "/Users/talha/Repos/VirtualEnvs/pelican-dev/lib/python2.7/site-packages/pelican-3.3-py2.7.egg/pelican/__init__.py", line 162, in run
    p.generate_context()
  File "/Users/talha/Repos/VirtualEnvs/pelican-dev/lib/python2.7/site-packages/pelican-3.3-py2.7.egg/pelican/generators.py", line 480, in generate_context
    signals.article_generator_finalized.send(self)
  File "/Users/talha/Repos/VirtualEnvs/pelican-dev/lib/python2.7/site-packages/blinker/base.py", line 267, in send
    for receiver in self.receivers_for(sender)]
  File "/Users/talha/Repos/pelican-plugins/neighbors/neighbors.py", line 49, in neighbors
    for subcategory, articles in generator.subcategories:
AttributeError: 'ArticlesGenerator' object has no attribute 'subcategories'
Merge pull request #149 from talha131/fix-neighbors

Neighbors plugin shouldn't require subcategories