Remove Gem

I think it has a potential to conflict with Homebrew files on macOS.

README.md

@@ -28,7 +28,6 @@ Just invoke `topgrade`. It will invoke the following steps:
   * [Plug](https://github.com/junegunn/vim-plug)
 * Upgrade NPM globally installed packages
 * Upgrade Atom packages
-* Upgrade RubyGems globally installed packages
 * *Linux*: Invoke the system package manager:
   * *Arch*: Invoke [yay](https://github.com/Jguer/yay) or fall back to pacman
   * *Redhat based*: Invoke `yum upgrade`

src/main.rs

@@ -125,11 +125,6 @@ fn main() -> Result<(), Error> {
         }
     }
 
-    if let Ok(gem) = which("gem") {
-        terminal.print_separator("RubyGems");
-        run_gem(&gem).report("RubyGems", &mut reports);
-    }
-
     if let Ok(npm) = which("npm") {
         terminal.print_separator("Node Package Manager");
         run_npm(&npm).report("Node Package Manager", &mut reports);

src/steps.rs

@@ -73,16 +73,6 @@ pub fn run_vim(
     Ok(())
 }
 
-pub fn run_gem(gem: &PathBuf) -> Result<(), failure::Error> {
-    Command::new(&gem)
-        .args(&["update"])
-        .spawn()?
-        .wait()?
-        .check()?;
-
-    Ok(())
-}
-
 pub fn run_npm(npm: &PathBuf) -> Result<(), failure::Error> {
     Command::new(&npm)
         .args(&["update", "-g"])