Remove explicit dependency on Rake

This isn't needed by default—it's only needed if users want to use the
Rake integration.

Remove it so we can avoid issues like #313.

Gemfile

@@ -4,6 +4,7 @@ source 'https://rubygems.org'
 
 gemspec
 
+gem 'rake' # For testing the Rake integration
 gem 'rspec', '~> 3.8'
 gem 'rspec-its', '~> 1.0'
 

README.md

@@ -341,7 +341,8 @@ Git hook manager, [overcommit](https://github.com/sds/overcommit).
 
 ## Rake Integration
 
-To execute `haml-lint` via a [Rake](https://github.com/ruby/rake) task, add the
+To execute `haml-lint` via a [Rake](https://github.com/ruby/rake) task, make
+sure you have `rake` included in your gem path (e.g. via `Gemfile`) add the
 following to your `Rakefile`:
 
 ```ruby

haml_lint.gemspec

@@ -25,7 +25,6 @@ Gem::Specification.new do |s|
 
   s.add_dependency 'haml', '>= 4.0', '< 5.2'
   s.add_dependency 'rainbow'
-  s.add_dependency 'rake', '>= 10', '< 14'
   s.add_dependency 'rubocop', '>= 0.50.0'
   s.add_dependency 'sysexits', '~> 1.1'
 end