Upgrade yajl-ruby and rake.

Dependabot said these dependencies contain security vulnerabilities,
so let's upgrade them.

Gemfile.lock

@@ -16,13 +16,13 @@ GEM
   specs:
     git (1.3.0)
     power_assert (0.4.1)
-    rake (12.0.0)
+    rake (13.0.1)
     rake-compiler (0.9.9)
       rake
     test-unit (3.2.3)
       power_assert
     thoughtbot-shoulda (2.11.1)
-    yajl-ruby (1.3.0)
+    yajl-ruby (1.3.1)
 
 PLATFORMS
   ruby