Change default ruby version from 2.3.4 to 2.4.1 [#143721855]
Change default ruby version from 2.3.4 to 2.4.1 [#143721855]

Signed-off-by: Keaty Gross <ba27100af1ae477f2abedd28f15b1dd208b1f48a@pivotal.io>

cloudfoundry/ruby-buildpack,cloudfoundry/ruby-buildpack,ekaranto/rails4-bluemix-buildpack,ekaranto/rails4-bluemix-buildpack,cloudfoundry/ruby-buildpack,cloudfoundry/ruby-buildpack,ekaranto/rails4-bluemix-buildpack,ekaranto/rails4-bluemix-buildpack,cloudfoundry/ruby-buildpack