Use activated environment by RVM

[vinistock]

Motivation

Closes #1863, closes #1866, closes #1858

There were two issues with our previous implementation of RVM activation:

1. For some reason, when I first tested, rvm-auto-ruby was not setting environment variables as expected. I must have done something wrong, because it does set variables properly. This is really important for RVM because it overrides GEM_HOME and GEM_PATH, which means we can't rely on Ruby's defaults
2. You can install RVM under /usr/local/rvm and not just ~/.rvm

Implementation

1. Added a method to search for the RVM installation, accounting for the other directory we missed
2. Switched the implementation to basically match what we do for Mise and Shadowenv. We just invoke RVM and let it set the environment

Automated Tests

Changed the existing tests.

[andyw8]

(not tophatted since I don't have RVM available)