Add fix for old vims

Old vim versions did not support both unicode() and str() in python2 bindings in 
a number of places.

Fixes #829

powerline/bindings/vim/__init__.py

@@ -90,7 +90,7 @@ else:
 
 if hasattr(vim, 'options'):
 	def vim_getbufoption(info, option):
-		return info['buffer'].options[option]
+		return info['buffer'].options[str(option)]
 else:
 	def vim_getbufoption(info, option):  # NOQA
 		return getbufvar(info['bufnr'], '&' + option)