Don't throw an error when a package is outdated

When a release is no longer available on pypi checkmyreqs throws an exception.

Example for django-extensions==1.1:

```python
Traceback (most recent call last):
  File "~/.virtualenvs/momox/bin/checkmyreqs", line 189, in <module>
    main()
  File "~/.virtualenvs/momox/bin/checkmyreqs", line 184, in main
    check_packages(packages, args.python)
  File "~/.virtualenvs/momox/bin/checkmyreqs", line 81, in check_packages
    supported_pythons = get_supported_pythons(package_info)
  File "~/.virtualenvs/momox/bin/checkmyreqs", line 131, in get_supported_pythons
    for c in classifiers:
TypeError: 'NoneType' object is not iterable
```

Available package releases are ['1.4.4', '1.4.3', '1.4.2', '1.4.1', '1.4.0', '1.3.11', '1.3.10', '1.3.9', '1.3.8', '1.3.7', '1.3.6', '1.3.5', '1.3.4', '1.3.3', '1.3.2', '1.3.1', '1.3.0', '1.2.5', '1.2.4', '1.2.3', '1.2.2', '1.2.1', '1.2.0'] and '1.1' is not there anymore.

A new message type eg. "package outdated" would be probably better, but it is relatively hard to implement this (comparing versions is tricky).