Fix rspec check

If you use rspec-rails, it's possible for `RSpec` to be defined, but not
for RSpec.configure to be available yet. This explicitly checks for that
method.

Author: technicalpickles

lib/singed.rb

@@ -53,4 +53,4 @@ def filter_line(line)
 
 require 'singed/kernel_ext'
 require 'singed/railtie' if defined?(Rails::Railtie)
-require 'singed/rspec' if defined?(RSpec)
+require 'singed/rspec' if defined?(RSpec) && RSpec.respond_to?(:configure)