docs: set pillow requirements for version already present on readthedocs.org

That way no uninstall is attempted, and no (failed) installation of an older
version crashes the builds.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

requirements-docs.txt

-# All pip installable requirements pinned for Travis CI
+# All pip installable requirements pinned for readthedocs.org
 fabric==1.10.0
 pystache==0.5.4
 requests==1.2.3
 PyYAML==3.11
-Pillow==2.2.1
+Pillow==2.6.1
 aexpect==1.0.0