Use lastest steep and remove `Grepx` definition

[hoshinotsuyoshi]

PR for https://github.com/pocke/rubocop-grep/issues/4

Steps to reproduce the problem

https://github.com/pocke/rubocop-grep/issues/4

Steps to reproduce the problem

1. Check out the https://github.com/ydah/sandbox-rubocop-grep
2. Run bundle install.
3. Run bundle exec rubocop.

Steps to verify this pull request

1. Check out the https://github.com/ydah/sandbox-rubocop-grep
2. Install this branch :

--- a/Gemfile
+++ b/Gemfile
@@ -1,6 +1,7 @@
 # frozen_string_literal: true

 source 'https://rubygems.org'
+git_source(:github) { |repo| "https://github.com/#{repo}.git" }

 gem 'rubocop'
-gem 'rubocop-grep'
+gem 'rubocop-grep', github: 'hoshinotsuyoshi/rubocop-grep', branch: 'bundle-update-steep-and-remove-grepx'

3. Run bundle exec rubocop.

$ bundle exec rubocop --cache=false -d

...snip....

Offenses:

target.rb:3:1: C: Grep/Grep: Do not refer ENV directly.
ENV
^^^

2 files inspected, 1 offense detected
Finished in 0.07266599999275059 seconds