pin pandas version to 0.23.4

trying to fix a problem with python 3.4 and pandas 0.24.0:

```
      File "/usr/lib/python3.4/site-packages/pkg_resources/__init__.py", line 348, in get_provider
        __import__(moduleOrReq)
    ImportError: No module named 'numpy'
```

ippisite/requirements-core.txt

@@ -7,7 +7,7 @@ django-formtools
 django-debug-toolbar
 django-allauth
 # import scripts
-pandas
+pandas==0.23.4
 # web services
 bioservices
 bioblend