Add Pillow to test-requirements.txt

Pillow isn't directly needed but it's pulled in via:
    Collecting Pillow (from blockdiag>=1.5.0->sphinxcontrib-blockdiag>=0.5.5

So cap as per global-requirements until https://launchpad.net/bugs/1501995
is properly fixed

Change-Id: Ifb8fe54d02a52f7be9c1c480da4d42c73bd9bdae
diff --git a/test-requirements.txt b/test-requirements.txt
index c68b2db..4ae3eb3 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -2,6 +2,11 @@
 
 coverage>=3.6
 sphinx>=1.1.2,!=1.2.0,!=1.3b1,<1.3
+# NOTE(tonyb) Pillow isn't directly needed but it's pulled in via
+# Collecting Pillow (from blockdiag>=1.5.0->sphinxcontrib-blockdiag>=0.5.5
+# So cap as per global-requirements until https://launchpad.net/bugs/1501995
+# is properly fixed
+Pillow>=2.4.0,<3.0.0 # MIT
 sphinxcontrib-blockdiag>=0.5.5
 discover
 fixtures>=0.3.14