Fix build error on Python 2.6

This was seen on winbot[*]:

  ValueError: path 'docs/_build/' cannot end with '/'

with a traceback ending in "c:\Python26_32\lib\distutils\util.py", line
201, in convert_path.

[*] http://winbot.zope.org/builders/transaction_py_265_32/builds/845/steps/buildout/logs/stdio

I cannot reproduce the error locally, using Python 2.6.9.

MANIFEST.in

@@ -13,7 +13,7 @@ global-exclude *.pyo
 global-exclude *.so
 global-exclude coverage.xml
 
-prune docs/_build/
+prune docs/_build
 
 include .coveragerc
 include .travis.yml