Use bundler/setup instead of Bunlder.require in multiline_repl (#860)

Bundler.require requires all gems including gems multiline_repl doesn't need.
This will fix ci failure in ruby-2.7 caused by warning message displayed while requiring power_assert.

Author: tompng

test/reline/yamatanooroti/multiline_repl

@@ -1,8 +1,6 @@
 #!/usr/bin/env ruby
 
-
-require 'bundler'
-Bundler.require
+require 'bundler/setup'
 
 require 'reline'
 require 'optparse'