detect file presence correctly

This is what happens when I try to install the facebook 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.