spec/helper.rb -> spec/spec_helper.rb

`helper.rb` is a legacy name from Bacon days. Since we use RSpec, the convention
is to use `spec/spec_helper.rb`. Applying the principle of least astonishment
here.

.rspec

@@ -1,2 +1,2 @@
 --color
---require helper
+--require spec_helper

spec/code_spec.rb

@@ -65,11 +65,11 @@ describe Pry::Code do
       end
 
       it 'finds files in a relative directory with `.rb` extension' do
-        expect(Pry::Code.from_file('../helper.rb').code_type).to eq :ruby
+        expect(Pry::Code.from_file('../spec_helper.rb').code_type).to eq :ruby
       end
 
       it 'finds files in a relative directory with `.rb` omitted' do
-        expect(Pry::Code.from_file('../helper').code_type).to eq :ruby
+        expect(Pry::Code.from_file('../spec_helper').code_type).to eq :ruby
       end
 
       it "doesn't confuse files with the same name, but without an extension" do

spec/ring_spec.rb

@@ -1,5 +1,3 @@
-require_relative 'helper'
-
 describe Pry::Ring do
   let(:ring) { described_class.new(3) }