Fix IOError when files listed under VCS are missing (#32)

This produces some nonsensical warnings, e.g.

    listing source files under version control: 11 files and directories
    building an sdist: check-manifest-0.22.dev0.tar.gz: 10 files and directories
    some files listed as being under source control are missing:
      LICENCE.rst
    copying source files to a temporary directory
    building a clean sdist: check-manifest-0.22.dev0.tar.gz: 10 files and directories
    files in version control do not match the sdist!
    missing from sdist:
      LICENCE.rst
    suggested MANIFEST.in rules:
      include *.rst

No, stupid computer, adding 'include *.rst' to MANIFEST.in is definitely
not going to help resurrect deleted files.