Drop ruby 2.0.0 support and add 2.3
Drop ruby 2.0.0 support and add 2.3

ProctorU/letter_opener_web,fgrehm/letter_opener_web,ProctorU/letter_opener_web,ProctorU/letter_opener_web,fgrehm/letter_opener_web,fgrehm/letter_opener_web,fgrehm/letter_opener_web,ProctorU/letter_opener_web