Relax pillow version restriction

It allows to build with Pillow 6.x.

CONTRIBUTORS.rst

@@ -365,6 +365,7 @@ Contributors
 * dthompson86
 * Jason Dilworth
 * Deniz Dogan
+* Po-Chuan Hsieh
 
 Translators
 ===========

setup.py

@@ -27,7 +27,7 @@ install_requires = [
     "django-treebeard>=4.2.0,<5.0",
     "djangorestframework>=3.7.4,<4.0",
     "draftjs_exporter>=2.1.5,<3.0",
-    "Pillow>=4.0.0,<6.0",
+    "Pillow>=4.0.0,<7.0.0",
     "beautifulsoup4>=4.5.1,<4.6.1",
     "html5lib>=0.999,<2",
     "Unidecode>=0.04.14,<2.0",