BLD: Fix problem with setup.py and setuptools (#637)

Fixes #631, thanks to @ikirker for the report and the suggested patch.

setup.py

@@ -42,17 +42,17 @@
 
 skbio_2 = "scikit-bio >= 0.4.1, < 0.5.0"
 skbio_3 = "scikit-bio >= 0.4.1"
-base = {"numpy >= 1.7", "scipy >= 0.17.0", "click", "pandas",
-        skbio_2, "jinja2 >= 2.9", "future"}
-doc = {"Sphinx < 1.6", "sphinx-bootstrap-theme"}
-test = {"nose >= 0.10.1", "pep8", "flake8"}
-all_deps = base | doc | test
+base = ["numpy >= 1.7", "scipy >= 0.17.0", "click", "pandas",
+        skbio_2, "jinja2 >= 2.9", "future"]
+doc = ["Sphinx < 1.6", "sphinx-bootstrap-theme"]
+test = ["nose >= 0.10.1", "pep8", "flake8"]
+all_deps = base + doc + test
 
 # prevent python2 from trying to install skbio >= 0.5.0 (which only works in
 # PY3K)
 if sys.version_info.major == 3:
     base.remove(skbio_2)
-    base.add(skbio_3)
+    base.append(skbio_3)
 
 setup(
     name='emperor',