Pre-install Ruby 2.6, 2.5, 2.4

[jankeromnes]

Should address gitpod-io/gitpod#353 somewhat.

This will not make rvm install 2.X persistent, however since 2.4, 2.5, and 2.6 will already be pre-installed, you can simply specify this in your .gitpod.yml:

tasks:
- before: rvm use 2.4

This will make sure that your terminals always use Ruby 2.4, even after you restart your workspace.

[jankeromnes]

@bwl21 could you please take a look at these Ruby changes, and let me know if they look good to you?

It pre-installs 2.4, 2.5 and 2.6 (latest stable patch for each), and also removes the RUBY_VERSION env variable, which didn't seem useful.

[jankeromnes]

Ok, let's try this.