Version Bump v3.6.3: sendgrid#243 Update deprecated Heroku command

Version Bump v3.6.2: Pull sendgrid#240 Add six to requirements.txt

Version Bump v3.6.1: Pull sendgrid#246 Typo 'user' for 'usr'

Version Bump v3.6.0: sendgrid#234 Substitutions allow non-strings for…

… values

Version Bump v3.5.0: sendgrid#233 Allow dict to be passed to add_headers

Version Bump v3.4.0: Support larger files, up to 20MB for Incoming Parse

Version Bump v3.3.1: Inbound Parse fixes

Version Bump v3.3.0: Pull sendgrid#212 - allow for custom Inbound Par…

…se config.yml

Version Bump v3.2.3: Pull sendgrid#211, updates to inbound parse

Version Bump v3.2.2: Added TOC and USE_CASES.md