Version Bump: v4.2.0: Pull sendgrid#318 Add ability to reset request …

…headers on client attribute

Version Bump v4.1.0: Pull sendgrid#314 Add ability to impersonate sub…

…user

Version Bump v4.0.0: BREAKING CHANGE sendgrid#244

Version Bump v3.6.5: sendgrid#300 Exclude test package

Version Bump v3.6.4: sendgrid#250 Improve code quality

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