emails should have html and plain text versions

see emails from r-hub as well as from travis-ci for how that's done. travis e.g.,

Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
# vs. 
Content-Type: text/html; charset=utf-8
Content-Transfer-Encoding: quoted-printable

in sendgrid-ruby gem, it looks like we can do something like

x = Content.new(type: 'text/html', value: '<html>some string</html>')
x = SendGrid::Content.new(type: 'text/plain', value: "another string")

sckott / cchecksapi

emails should have html and plain text versions #54