[Fix #104] Exclude Rails-independent bin/bundle by default

[koic]

Resolves #104.

bin/bundle was generated by Bundler. And Bundler doesn't depend on Active Support (Rails).

This PR prevents the following error.

% cd path/to/new-rails-app
% bin/rails -v
Rails 6.0.0.rc2
% bundle exec rubocop -a --only Rails/Present
% g diff
diff --git a/bin/bundle b/bin/bundle
index 4f5e057..fa7b83b 100755
--- a/bin/bundle
+++ b/bin/bundle
@@ -39,7 +39,7 @@ m = Module.new do

   def gemfile
     gemfile = ENV["BUNDLE_GEMFILE"]
-    return gemfile if gemfile && !gemfile.empty?
+    return gemfile if gemfile.present?

     File.expand_path("../../Gemfile", __FILE__)
   end
% ./bin/bundle
Traceback (most recent call last):
        2: from ./bin/bundle:101:in `<main>'
        1: from ./bin/bundle:71:in `load_bundler!'
./bin/bundle:42:in `gemfile': undefined method `present?' for
   nil:NilClass (NoMethodError)

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.