Update setup.py

psycopg2 keeps failing because it sees the latest version of postgresql as 10.5 and cannot interpret that as a version number correctly. 

Changed the version of psycopg2 to 2.7.5 as per psycopg/psycopg2#489 because the maintainers have no desire to backport the fix. Here's hoping it doesn't fail now for some other reason on postgresql 10.5

setup.py

@@ -20,7 +20,7 @@
                         'numpy==1.9.0',
                         'SQLAlchemy==1.0.12',
                         'pyhocon==0.3.29',
-                        'psycopg2==2.6.1',],
+                        'psycopg2==2.7.5',],
     include_package_data = True,
     entry_points = {
           'console_scripts': [