SourceImport.update() is really GithubObject.update(), which just calls
GET on the object's URL. Refetching a source import requires an
additional header, so change GithubObject.update() to pass any it is
given, and add a test for SourceImport.update()
Fixes#1297
With the Python 2.7 deadline fast approaching, modernize the codebase
making use of the modernize module to switch to using six, as well as
other upcoming features, such as absolute imports . Stop using 2to3
for Travis, yay!
Steve Kowalik