Use bundler to make sure which version of simplecov-html we run

Author: amatsuda

spec/coverage_for_eval_spec.rb

@@ -16,7 +16,7 @@
     end
 
     context "foo" do
-      let(:command) { "ruby eval_test.rb" }
+      let(:command) { "bundle e ruby eval_test.rb" }
 
       it "records coverage for erb" do
         expect(@stdout).to include(" 2 / 3 LOC")