MJIT: Use install = true for bundler/inline

It prints a `bundle install`-like output, which seems more useful than
a silent output.

tool/mjit/bindgen.rb

@@ -4,7 +4,7 @@
 ENV['GEM_HOME'] = File.expand_path('./.bundle', __dir__)
 require 'rubygems/source'
 require 'bundler/inline'
-gemfile do
+gemfile(true) do
   source 'https://rubygems.org'
   gem 'ffi-clang', '0.7.0', require: false
 end