add 'require: false' to readme instructions
add 'require: false' to readme instructions

Solves https://github.com/spilin/capistrano3-postgres/issues/1
RyanMacG/capistrano3-postgres,spilin/capistrano3-postgres,joshuapaling/capistrano3-postgres,hoppinger/capistrano3-postgres,ypadlyak/capistrano3-postgres