[Bugfix] Make prompt_rvm list gemset

[MartyBeGood]

The RVM Prompt segment doesn't display the used gemset, only the used ruby version.

The documentation for rvm-prompt says that

• v returns the version (e.g. 2.6.0)
• p returns the patchlevel (e.g. -rc1)
• g returns the gemset (e.g. @foo)

So

• rvm-prompt v p (current master) would return 2.6.0-rc1
• we should be using rvm-prompt v p g which would return 2.6.0-rc1@foo

Also we don't need to strip a ruby- from the output string. That's the interpreter identifier (for MRI) that would be included with the i flag.