Update `requests` library to 1.2.3
Use oauthlib directly for LTI module
Pass hostname to LTI XModule

Necessary for RFC compliance with oauthlib
Response.json is now a callable
Add default launch url for lti.
Merge branch 'db/update-requests-lib' of github.com:edx/edx-platform into db/update-requests-lib
Stub headers if scheme not provided.
reformat for readability
Merge pull request #880 from edx/db/update-requests-lib

Update `requests` library to 1.2.3