title: "ValueError: labels ['timestamp'] not contained in axis" error
body: <p>I have this code ,i want to remove the column 'timestamp' from the file :<a href="https://www.dropbox.com/s/4cksq4dwgbaqd07/u.data?dl=0" rel="noreferrer">u.data</a> but can't.It shows the error<br>
  "ValueError: labels ['timestamp'] not contained in axis"
  How can i correct it</p>
  
  <pre><code>import numpy as np
  import pandas as pd
  import matplotlib.pyplot as plt 
  plt.rc("font", size=14)
  from sklearn.linear_model import LinearRegression
  from sklearn.linear_model import Ridge
  from sklearn.cross_validation import KFold
  from sklearn.cross_validation import train_test_split
  
  
  
  data = pd.read_table('u.data')
  data.columns=['userID', 'itemID','rating', 'timestamp']
  data.drop('timestamp', axis=1)
  
  
  N = len(data)
  print data.shape
  print list(data.columns)
  print data.head(10)
  </code></pre>