Update rake to v13

This comes standard in Ruby v2.7.2, so we don't need an extra gem. It
fixes a number of minor bugs:
https://github.com/ruby/rake/blob/master/History.rdoc

Gemfile.lock

@@ -922,7 +922,7 @@ GEM
       thor (>= 0.20.3, < 2.0)
     rainbow (3.0.0)
     raindrops (0.19.1)
-    rake (12.3.3)
+    rake (13.0.1)
     rb-fsevent (0.10.4)
     rb-inotify (0.10.1)
       ffi (~> 1.0)