registry/index.py: Add '# nopep8' tags to long URL lines

This makes the lines even longer, but avoids tox failures:

  /.../registry/index.py:110:80: E501 line too long (116 > 79 characters)
  /.../registry/index.py:111:80: E501 line too long (112 > 79 characters)

Alternatively, we could ignore E501 entirely (with an entry in
tox.ini's flake8.ignore), but Sam Alba prefers ignoring just those
lines (and not all long lines) [1].

[1]: https://github.com/dotcloud/docker-registry/pull/247#issuecomment-36075734