Update distributions and Ruby

Depends on #90.

website       ubuntu:18.04          DONE (to get therubyracer and Latex)
sharelatex    ubuntu:18.04          DONE
quiz-db       mongo                 (official) wait till after the semester
sharelatex-db mongo                 (official) wait till after the semester
cndumper      ruby:2.6.x-alpine3.8
quiz          ruby:2.6.x-alpine3.8  DONE
quiz-parser                         lib
getpdf        ruby:2.6.x-alpine3.8  DONE
enote-core                          lib
enote-pdfjs                         lib

Looks like we can't stay with MongoDB 3.0.12 for the quiz anymore. https://hub.docker.com/_/mongo/

Sharelatex uses mongo:4.0.0 so that shouldn't be a problem. I'll upgrade Sharelatex as one of the last things before feature freeze. Ideally the Quiz is upgraded to use the same version of MongoDB as Sharelatex.

Ubuntu 18.04 uses Ruby 2.5.