Failing test for platform='gittip' in tips.json

This is a regression from the rename.
Fix tips.json to still honor platform='gittip'
Reformat a bit while we're in here

We upped to 100-chars since this file was written.
Add note in README
Merge pull request #3035 from gratipay/platform-gittip

tips.json API should still work with platform='gittip'