Bump Ruby to 2.5.3

What

• Bump Ruby to 2.5.3. cf. omnibus-gitlab!2806 (merged)
• Pin Bundler 1.13.6 as well as in ruby_docker like other builds
• Replace deprecated "--no-ri --no-rdoc" with "--no-document". cf. omnibus-gitlab!3050 (merged)

Bump Ruby to 2.5.3

before

$ docker run --rm registry.gitlab.com/gitlab-org/gitlab-omnibus-builder/debian_9:0.0.29 ruby -v
ruby 2.4.4p296 (2018-03-28 revision 63013) [x86_64-linux]

after

$ docker run --rm registry.gitlab.com/gitlab-org/gitlab-omnibus-builder/staging/debian_9:ruby-2-5-3 ruby -v
ruby 2.5.3p105 (2018-10-18 revision 65156) [x86_64-linux]

Closes #15 (closed)