Update rubocop-rspec requirement from ~> 2.11.1 to ~> 2.20.0 (#457)

Updates the requirements on [rubocop-rspec](https://github.com/rubocop/rubocop-rspec) to permit the latest version.
- [Release notes](https://github.com/rubocop/rubocop-rspec/releases)
- [Changelog](https://github.com/rubocop/rubocop-rspec/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rubocop/rubocop-rspec/compare/v2.11.1...v2.20.0)

---
updated-dependencies:
- dependency-name: rubocop-rspec
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

1 files changed, 7 insertions, 5 deletions

diff --git a/spec/converter_spec.rb b/spec/converter_spec.rb
index 623acd9..b19406d 100644
--- a/spec/converter_spec.rb
+++ b/spec/converter_spec.rb
@@ -5,15 +5,15 @@ require_relative 'spec_helper'
 describe Asciidoctor::Epub3::Converter do
   describe '#convert' do
     it 'converts empty file to epub without exceptions' do
-      to_epub fixture_file('empty.adoc')
+      expect(to_epub(fixture_file('empty.adoc'))).not_to be_nil
     end
 
     it 'converts empty file to mobi without exceptions' do
-      to_mobi fixture_file('empty.adoc')
+      expect(to_mobi(fixture_file('empty.adoc'))).not_to be_nil
     end
 
     it 'converts empty heredoc document to epub without exceptions' do
-      to_epub ''
+      expect(to_epub('')).not_to be_nil
     end
 
     it 'converts minimal heredoc document to epub without exceptions' do
@@ -262,21 +262,23 @@ describe Asciidoctor::Epub3::Converter do
     end
 
     it "doesn't crash when sees inline toc" do
-      to_epub <<~EOS
+      book = to_epub <<~EOS
         = Title
 
         toc::[]
       EOS
+      expect(book).not_to be_nil
     end
 
     it 'does not crash when sees inline pass macro `m` with an icon' do
-      to_epub <<~EOS
+      book = to_epub <<~EOS
         = Test
         :icons: font
         :call-to-action: pass:m,q[icon:hand-o-right[] *Call to Action*]
 
         {call-to-action}
       EOS
+      expect(book).not_to be_nil
     end
 
     it 'supports video' do