Try to fix Python 2.6 build

Apparently the latest markdown no longer works against py2.6; Travis
fails on trying to import importlib.  I added a dependency on the PyPI
backport to see if that helps for now.

setup.py

@@ -1,4 +1,18 @@
 from setuptools import setup, find_packages
+
+import sys
+
+deps = [
+    'SQLAlchemy>=0.9.7',
+    'whoosh>=2.5,<2.7',
+    'markdown',
+    'construct',
+    'six>=1.9.0',
+]
+if sys.version_info < (2, 7):
+    # We don't actually use this, but markdown does
+    deps.append('importlib')
+
 setup(
     name = 'Pokedex',
     version = '0.1',
@@ -7,13 +21,7 @@ setup(
     package_data = {
         'pokedex': ['data/csv/*.csv']
     },
-    install_requires=[
+    install_requires=deps,
-        'SQLAlchemy>=0.9.7',
-        'whoosh>=2.5,<2.7',
-        'markdown',
-        'construct',
-        'six>=1.9.0',
-    ],
 
     entry_points = {
         'console_scripts': [