chore: Make init uses bundle install (getsentry#3756)

Make init now uses bundle install, which installs the gems specified in
the gemfile, instead of bundle update, which updates the gems to the
latest version specified in the gemfile.

Makefile

@@ -4,7 +4,7 @@ init: setup-git
 	brew bundle
 	rbenv install --skip-existing
 	rbenv exec gem update bundler
-	rbenv exec bundle update
+	rbenv exec bundle install
 
 .PHONY: setup-git
 setup-git: