fix(rubocop): support projects that do not use bundler

lua/lspconfig/server_configurations/rubocop.lua

@@ -2,10 +2,17 @@ local util = require 'lspconfig.util'
 
 return {
   default_config = {
-    cmd = { 'bundle', 'exec', 'rubocop', '--lsp' },
+    cmd = { 'rubocop', '--lsp' },
     filetypes = { 'ruby' },
     root_dir = util.root_pattern('Gemfile', '.git'),
   },
+  on_new_config = function(config, root_dir)
+    -- prepend 'bundle exec' to cmd if bundler is being used
+    local gemfile_path = util.path.join(root_dir, 'Gemfile')
+    if util.path.exists(gemfile_path) then
+      config.cmd = { 'bundle', 'exec', unpack(config.cmd) }
+    end
+  end,
   docs = {
     description = [[
 https://github.com/rubocop/rubocop