title: concat() got an unexpected keyword argument 'join_axes'
body: <p>I am trying to use pymc3 in an ipynb on Google Colab.</p>
  
  <p>Here is my code:</p>
  
  <pre><code>regression_conjugate = pm.Model()
  with regression_conjugate:
    sigma2 = pm.InverseGamma("sigma2",alpha = 0.5*nu0,beta=0.5*lam0)
    sigma = pm.math.sqrt(sigma2)
    a = pm.Normal("a",mu = b0[0],sd = sigma*sd0[0])
    b = pm.Normal("b",mu = b0[1],sd = sigma*sd0[1])
    y_hat = a+b*x
    likelihood = pm.Normal("y",mu = y_hat,sd = sigma,observed = y)
  
  n_draws = 50
  n_chains = 4
  n_tune = 1000
  with regression_conjugate:
    trace = pm.sample(draws = n_draws, chains=n_chains,tune=n_tune,random_seed=123)
  
  print(pm.summary(trace))
  </code></pre>
  
  <p>However, this outputs the following:</p>
  
  <pre><code>TypeError                                 Traceback (most recent call last)
  &lt;ipython-input-44-4e1a1fef1a74&gt; in &lt;module&gt;()
       26   trace = pm.sample(draws = n_draws, chains=n_chains,tune=n_tune,random_seed=123)
       27 
  ---&gt; 28 print(pm.summary(trace))
  
  TypeError: concat() got an unexpected keyword argument 'join_axes'
  </code></pre>
  
  <p>please let me know if anyone who understands.</p>
  
  <p>pymc3 : 3.7</p>
  
  <p>pandas : 1.0.3</p>