Turn on simplecov to measure code coverage

Gemfile.lock

@@ -2,6 +2,7 @@ GEM
   remote: https://rubygems.org/
   specs:
     diff-lcs (1.5.0)
+    docile (1.4.0)
     psych (5.1.0)
       stringio
     rake (13.0.6)
@@ -21,6 +22,12 @@ GEM
       diff-lcs (>= 1.2.0, < 2.0)
       rspec-support (~> 3.12.0)
     rspec-support (3.12.1)
+    simplecov (0.22.0)
+      docile (~> 1.1)
+      simplecov-html (~> 0.11)
+      simplecov_json_formatter (~> 0.1)
+    simplecov-html (0.12.3)
+    simplecov_json_formatter (0.1.4)
     stringio (3.0.7)
     syntax (1.2.2)
 
@@ -32,6 +39,7 @@ DEPENDENCIES
   rdoc
   redcarpet
   rspec
+  simplecov
   syntax
 
 BUNDLED WITH