title: Delete Pandas DataFrame row where column value is < 0
body: <p>I already read the answers in <a href="https://stackoverflow.com/questions/18172851/deleting-dataframe-row-in-pandas-based-on-column-value">this</a> thread but it doesn't answer my exact problem.
  My DataFrame looks like this</p>
  
  <pre><code>                      Lady in the Water  The Night Listener  Just My Luck  Correlation
  Claudia Puig                    NaN                 4.5           3.0     0.893405
  Gene Seymour                    3.0                 3.0           1.5     0.381246
  Jack Matthews                   3.0                 3.0           NaN     0.662849
  Lisa Rose                       2.5                 3.0           3.0     0.991241
  Michael Phillips                2.5                 4.0           NaN    -1.000000
  Mick LaSalle                    3.0                 3.0           2.0     0.924473
  </code></pre>
  
  <p>and i want to delete Michael Phillips' row since his correlation value is below zero.
  As said, I tried different combinations of <code>df = df[df.Correlation]</code> and <code>df = df.drop()</code>but couldn't find anything that worked.</p>