Remove incompatible --color CLI option

The option was removed in RSpec 4, see rspec/rspec-core#2864
Follow-up to #2458

Author: pirj

Rakefile

@@ -19,7 +19,6 @@ end
 desc "Run all examples"
 RSpec::Core::RakeTask.new(:spec) do |t|
   t.ruby_opts = %w[-w]
-  t.rspec_opts = %w[--color]
 end
 
 Cucumber::Rake::Task.new(:cucumber) do |t|