[Sphinx] Fix the copyright year to always be the current year.

This is what we were doing manually anyway.

Author: jrose-apple

docs/conf.py

@@ -11,6 +11,7 @@
 # serve to show the default.
 
 import sys
+from datetime import date
 
 # If extensions (or modules to document with autodoc) are in another directory,
 # add these directories to sys.path here. If the directory is relative to the
@@ -40,7 +41,7 @@
 
 # General information about the project.
 project = u'Swift'
-copyright = u'2015, Apple Inc'
+copyright = unicode(date.today().year) + u', Apple Inc'
 
 # The version info for the project you're documenting, acts as replacement for
 # |version| and |release|, also used in various other places throughout the