Explicit gzip compression of fixture to make the test work with pandas<0.16.1.

In pandas>=0.16.1 gzip compression is automatically detected based on
the file_path string ending.

Author: dukebody

tests/test_dataframe_mapper.py

@@ -31,7 +31,7 @@ def iris_dataframe():
 
 @pytest.fixture
 def cars_dataframe():
-    return pd.read_csv("tests/test_data/cars.csv.gz")
+    return pd.read_csv("tests/test_data/cars.csv.gz", compression='gzip')
 
 
 def test_with_iris_dataframe(iris_dataframe):