Fix remaining RuboCop warnings

Author: eregon

spec/truffle/binding/of_caller_spec.rb

@@ -22,7 +22,7 @@ def binding_of_caller
   end
 
   it "gives read access to local variables at the call site" do
-    x = 14
+    x = 14 # rubocop:disable Lint/UselessAssignment
     binding_of_caller.local_variable_get(:x).should == 14
   end
 
@@ -33,7 +33,7 @@ def binding_of_caller
   end
 
   it "works through #send" do
-    x = 14
+    x = 14 # rubocop:disable Lint/UselessAssignment
     Truffle::Binding.send(:of_caller).local_variable_get(:x).should == 14
   end
 

spec/truffle/boot/source_of_caller_spec.rb

@@ -26,7 +26,6 @@ def source_of_caller
   end
 
   it "works through #send" do
-    x = 14
     Truffle::Boot.send(:source_of_caller).should == __FILE__
   end
 

spec/truffle/truffleruby/atomic_reference_spec.rb

@@ -72,7 +72,7 @@
     reference = TruffleRuby::AtomicReference.new(:value)
     dumped = Marshal.dump(reference)
     loaded = Marshal.load(dumped)
-    reference.get.should == :value
+    loaded.get.should == :value
   end
 
 end