detect file presence correctly

[bhs]

This is what happens when I try to install the facebook ads SDK using pip:

$ sudo pip install facebookads
Downloading/unpacking facebookads
  Downloading facebookads-0.2.1.tar.gz (377kB): 377kB downloaded
  Running setup.py (path:/private/tmp/pip_build_root/facebookads/setup.py) egg_info for package facebookads
    Traceback (most recent call last):
      File "<string>", line 17, in <module>
      File "/private/tmp/pip_build_root/facebookads/setup.py", line 33, in <module>
        readme_filename = os.path.join(this_dir, readme_filename)
    NameError: name 'readme_filename' is not defined
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):

  File "<string>", line 17, in <module>

  File "/private/tmp/pip_build_root/facebookads/setup.py", line 33, in <module>

    readme_filename = os.path.join(this_dir, readme_filename)

NameError: name 'readme_filename' is not defined

----------------------------------------
Cleaning up...
Command python setup.py egg_info failed with error code 1 in /private/tmp/pip_build_root/facebookads
Storing debug log for failure in /Users/bhs/Library/Logs/pip.log

I believe the change here will address the above.

[facebook-github-bot]

Thank you for your pull request and welcome to our community. We require contributors to sign our Contributor License Agreement, and we don't seem to have you on file. In order for us to review and merge your code, please sign up at https://code.facebook.com/cla - and if you have received this in error or have any questions, please drop us a line at cla@fb.com. Thanks!

[bhs]

thank you, @facebook-github-bot :) I have done so.

[facebook-github-bot]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Facebook open source project. Thanks!

[ebzlo]

Thanks Ben!