Use ruby:2.3-slim tag for CI Dockerfile
Use ruby:2.3-slim tag for CI Dockerfile

- 2.3.1-slim is not supported anymore and used Debian 8
  Latest supported apt version of git is 2.1.4 and we need a newer version
  for go modules

cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci