Jinja2>=2.7 is actually needed.

With jinja2<2.7, cookiecutter fails with error:

  File "/usr/local/lib/python2.7/site-packages/cookiecutter/generate.py", line 181, in generate_files
    env = Environment(keep_trailing_newline=True)
TypeError: __init__() got an unexpected keyword argument 'keep_trailing_newline'