MAINT: set upper bound on natsort version

the default sorting function changed in natsort 4.0. setting an upper bound on
the required version to maintain the old default for consistency with prior
versions of QIIME. See
[scikit-bio/scikit-bio#930](scikit-bio/scikit-bio#930) for
more detail.

setup.py

@@ -433,6 +433,7 @@ def build_swarm():
       install_requires=['numpy >= 1.9.0',
                         'scipy >= 0.14.0',
                         'cogent == 1.5.3',
+                        'natsort < 4.0.0',
                         'matplotlib >= 1.1.0, != 1.4.2',
                         'pynast == 1.2.2', 'qcli >= 0.1.1, < 0.2.0', 'gdata',
                         'biom-format >= 2.1.4, < 2.2.0',