Added mention of Ruby 2.3 to README
Added mention of Ruby 2.3 to README

sclorg/s2i-ruby-container,bparees/sti-ruby,mfojtik/sti-ruby,openshift/sti-ruby,hhorak/sti-ruby,sclorg/s2i-ruby-container,openshift/sti-ruby,mfojtik/sti-ruby,hhorak/sti-ruby,bparees/sti-ruby,sclorg/s2i-ruby-container