MAINT: Add __future__ print function import.

We do print(stock) in this file, which happens to work in py2, but is
confusing.

zipline/data/loader.py

@@ -12,6 +12,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+from __future__ import print_function
 import os
 from collections import OrderedDict