Request raw data for gists that are truncated

Gist files greater than 1 MB are now truncated in the gist API.
https://developer.github.com/changes/2014-05-06-gist-api-now-truncating-large-files/.
Look for the truncated flag and request the full contents from the
raw_url if it's set.
Merge pull request #367 from jwass/gist_rerequest

Request raw data for gists that are truncated