Speed up Ruby installation

[BR0kEN-]

For the moment we're installing Ruby using rbenv install and this process can get some boost by avoiding generation the ri/RDoc documentation.

rbenv/ruby-build#1054 (comment) or https://coderwall.com/p/zrrxna/speed-up-building-ruby

To do

• Allow installing 2.5.0.
• Create the changelog record.
• Add the RUBY_CONFIGURE_OPTS="--disable-install-doc" environment variable to avoid building docs.

[BR0kEN-]

The original issue that is being discussed at rbenv/ruby-build#42.

[BR0kEN-]

Can we consider downloading the third-party hosted binaries from https://rvm.io/binaries/ubuntu/16.04/x86_64/ instead of compiling from source?