update info about PIL

DEPENDS.txt

@@ -46,6 +46,10 @@ functionality is only available with the following installed:
    The ``pyamg`` module is used for the fast `cg_mg` mode of random
    walker segmentation.
 
+* `Pillow <https://pypi.python.org/pypi/Pillow>`__
+   (or <`PIL http://www.pythonware.com/products/pil/>`__)
+   The ``Pillow`` (or equivalently ``PIL``) is used for Input/Output.
+
 Testing requirements
 --------------------
 * `Nose <https://nose.readthedocs.org/en/latest/>`__

skimage/io/_plugins/pil_plugin.py

@@ -6,7 +6,9 @@ try:
     from PIL import Image
 except ImportError:
     raise ImportError("The Python Image Library could not be found. "
-                      "Please refer to http://pypi.python.org/pypi/PIL/ "
+                      "Please refer to "
+                      "https://pypi.python.org/pypi/Pillow/ (or "
+                      "http://pypi.python.org/pypi/PIL/) "
                       "for further instructions.")
 
 from skimage.util import img_as_ubyte