Handle blank lines in requirements.in

Don't choke when `requirements.in` has a blank line.

```
$ pundler install
processing requirements.in
handling requirement:
Traceback (most recent call last):
  File "/Users/marca/dev/surveymonkey/profilesvc/src/profilesvc/py27.venv/bin/pundler", line 10, in <module>
    execfile(__file__)
  File "/Users/marca/dev/git-repos/pundler/bin/pundler", line 3, in <module>
    main()
  File "/Users/marca/dev/git-repos/pundler/pundler/core.py", line 165, in main
    args.func(args)
  File "/Users/marca/dev/git-repos/pundler/pundler/core.py", line 137, in install
    pundler.process_requirements(filename)
  File "/Users/marca/dev/git-repos/pundler/pundler/core.py", line 76, in process_requirements
    requirement = InstallRequirement.from_line(line, None)
  File "/Users/marca/dev/surveymonkey/profilesvc/src/profilesvc/py27.venv/lib/python2.7/site-packages/pip/req.py", line 172, in from_line
    return cls(req, comes_from, url=url, prereleases=prereleases)
  File "/Users/marca/dev/surveymonkey/profilesvc/src/profilesvc/py27.venv/lib/python2.7/site-packages/pip/req.py", line 70, in __init__
    req = pkg_resources.Requirement.parse(req)
  File "/Users/marca/dev/surveymonkey/profilesvc/src/profilesvc/py27.venv/lib/python2.7/site-packages/pip/_vendor/pkg_resources.py", line 2672, in parse
    raise ValueError("No requirements found", s)
ValueError: ('No requirements found', '')
```