fix word matches in transform_ipynb_uri

`\w` is never the right pattern, use `[\w-]`
fix url_rewrite_dict ordering

it was an OrderedDict created from a regular unordered dict,
defeating the purpose and making a random order.
handle GitHub tree urls
skip github blob redirect test

While we figure out the right fix.

The underlying GitHub redirect we relied upon no longer happens.
Merge pull request #274 from minrk/backslash-w-minus

fix word matches in transform_ipynb_uri