Add fixnum scalar bounds mutation for literal fixnu,s

Closes #23

lib/mutant/mutator/node/literal/fixnum.rb

@@ -28,7 +28,8 @@ module Mutant
           # @api private
           #
           def values
-            [0, 1, -children.first]
+            value = children.first
+            [0, 1, -value, value+1, value-1]
           end
 
         end # Fixnuma

spec/unit/mutant/mutator/node/literal/fixnum_spec.rb

@@ -6,7 +6,7 @@ describe Mutant::Mutator::Node::Literal, 'fixnum' do
   let(:source) { '10' }
 
   let(:mutations) do
-    %W(nil 0 1 #{random_fixnum} -10)
+    %W(nil 0 1 #{random_fixnum} -10 9 11)
   end
 
   before do