Revert "Huge performance improvement if you use `rvm` segment."

This reverts commit 643555a30c.

This does allow rvm to be detected even without using the depreciated
gemset settings.

powerlevel9k.zsh-theme

@@ -1365,10 +1365,10 @@ prompt_rspec_stats() {
 ################################################################
 # Segment to display Ruby Version Manager information
 prompt_rvm() {
-  local version_and_gemset=${rvm_env_string/ruby-}
+  local version_and_gemset=${$(rvm-prompt)/ruby-}
 
   if [[ -n "$version_and_gemset" ]]; then
-    "$1_prompt_segment" "$0" "$2" "grey35" "$DEFAULT_COLOR" "$version_and_gemset" 'RUBY_ICON'
+    "$1_prompt_segment" "$0" "$2" "240" "$DEFAULT_COLOR" "$version_and_gemset" 'RUBY_ICON'
   fi
 }