Enable warnings and fix some violations

.rspec

@@ -1,4 +1,5 @@
 --color
 --format documentation
 --backtrace
---default_path spec
+--order random
+--warnings

lib/rbnacl/hash.rb

@@ -57,7 +57,6 @@ def self.sha512(data)
     #
     # @return [String] The blake2b hash as raw bytes (Or encoded as per the second argument)
     def self.blake2b(data, options = {})
-      key         = options[:key]
       Blake2b.new(options).digest(data)
     end
   end

lib/rbnacl/self_test.rb

@@ -46,7 +46,6 @@ def box_common_test(box)
         passed         = false
         corrupt_ct     = ciphertext.dup
         corrupt_ct[23] = ' '
-        corrupt_ct
         box.decrypt(nonce, corrupt_ct)
       rescue CryptoError
         passed = true

spec/shared/box.rb

@@ -1,7 +1,5 @@
 # encoding: binary
 
-require 'spec_helper'
-
 shared_examples "box" do
   let(:nonce) { vector :box_nonce }
   let(:invalid_nonce) { nonce[0,12]  } # too short!