fixes #91: must install future for builtins

[TheJefe]

without this change, I get this issue until I manually install future

[ec2-user@operator ~]$ sudo amicleaner --full-report
Traceback (most recent call last):
  File "/usr/local/bin/amicleaner", line 11, in <module>
    load_entry_point('aws-amicleaner==0.2.2', 'console_scripts', 'amicleaner')()
  File "/usr/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 564, in load_entry_point
    return get_distribution(dist).load_entry_point(group, name)
  File "/usr/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 2662, in load_entry_point
    return ep.load()
  File "/usr/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 2316, in load
    return self.resolve()
  File "/usr/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 2322, in resolve
    module = __import__(self.module_name, fromlist=['__name__'], level=0)
  File "/usr/local/lib/python2.7/site-packages/amicleaner/cli.py", line 6, in <module>
    from builtins import input
ImportError: No module named builtins

fixes #91