Remove explicit require of rspec to fix builds

Not sure why this works, or is even possible? But I can reproduce locally that
all non-1.8 builds fail when using a bundler cache unless this line is removed.

My guess is that, when using the --path option to bundler, all gems are being
implicitly required, and requiring rspec a second time is somehow causing issues,
but that doesn't make much sense to me.

spec/spec_helper.rb

@@ -30,7 +30,6 @@ $LOAD_PATH.unshift lib_dir unless $LOAD_PATH.include? lib_dir
 require 'docile'
 
 require 'singleton'
-require 'rspec'
 
 RSpec.configure do |config|
   config.expect_with :rspec do |c|