title: Install Airflow - ImportError: No module named clsregistry
body: <p>Hi I am new to airflow and try to install it. And I can install airflow successfully but when I do an <code>airflow initdb</code>, the error is :</p>
  <pre><code>(airflow_virtualenv) C02CF6JRMD6R:bin ybao$ airflow initdb
  
      Traceback (most recent call last):   File &quot;/Users/ybao/airflow_virtualenv/bin/airflow&quot;, line 26, in &lt;module&gt;
          from airflow.bin.cli import CLIFactory   File &quot;/Users/ybao/airflow_virtualenv/lib/python2.7/site-packages/airflow/bin/cli.py&quot;, line 82, in &lt;module&gt;
          from airflow.www.app import (cached_app, create_app)   File &quot;/Users/ybao/airflow_virtualenv/lib/python2.7/site-packages/airflow/www/app.py&quot;, line 42, in &lt;module&gt;
          from airflow.www.blueprints import routes   File &quot;/Users/ybao/airflow_virtualenv/lib/python2.7/site-packages/airflow/www/blueprints.py&quot;, line 25, in &lt;module&gt;
          from airflow.www import utils as wwwutils   File &quot;/Users/ybao/airflow_virtualenv/lib/python2.7/site-packages/airflow/www/utils.py&quot;, line 40, in &lt;module&gt;
          import flask_admin.contrib.sqla.filters as sqlafilters   File &quot;/Users/ybao/airflow_virtualenv/lib/python2.7/site-packages/flask_admin/contrib/sqla/__init__.py&quot;, line 2, in &lt;module&gt;
          from .view import ModelView   File &quot;/Users/ybao/airflow_virtualenv/lib/python2.7/site-packages/flask_admin/contrib/sqla/view.py&quot;, line 18, in &lt;module&gt;
          from flask_admin.contrib.sqla.tools import is_relationship   File &quot;/Users/ybao/airflow_virtualenv/lib/python2.7/site-packages/flask_admin/contrib/sqla/tools.py&quot;, line 4, in &lt;module&gt;
          from sqlalchemy.ext.declarative.clsregistry import _class_resolver ImportError: No module named clsregistry
  </code></pre>
  <p>Where should I install the packages?</p>
  <p>Thanks!!</p>