Fix RUBY_VERSION to be set to current version used

[bkuhlmann]

Overview

When using Frum, I'm not seeing RUBY_VERSION set within the environment.

Screencast

ruby_version.mp4

Steps to Recreate

1. Install Frum.
2. Configure Frum for your shell (in my case, I'm using Bash).
3. Run ENV | grep RUBY, notice that RUBY_VERSION doesn't display in the list.

Desired Behavior

As one switches between different Ruby versions, it'd be nice to see RUBY_VERSION reflect the current version being used. This would match behavior of similar Ruby version managers.

Environment

• macOS: macOS 11.5.1
• Frum: 0.1.1
• Ruby: 3.0.2p107 (2021-07-07 revision 0db68f0233) [arm64-darwin20.5.0]

[CodeMeister]

Can we bump this up the priority list a bit?

When testing code against multiple version of ruby on a regular basis, being able to display the current ruby version in the shell prompt would be great.

If an environmental variable is too tricky, perhaps a shell command similar to RVM's approach might be simper.

[DcsMarionDickten]

Using 11.6 and frum 0.1.2, the RUBY_VERSION variable is displayed (on my machine) but contains the wrong value.