Update mocha to fix issue with Minitest compatibility layer

There's a number of deprecation warnings to work through related to
mocha updates in v2+, we'll get through those on a separate change.
https://github.com/freerange/mocha/blob/main/RELEASE.md#200

The main issue is with Minitest, fixed in v2.1:
https://github.com/freerange/mocha/blob/main/RELEASE.md#210

Also run `bundle update` on the main Gemfile to update all dependencies
there to latest.

Gemfile

@@ -19,7 +19,7 @@ group :test do
   gem "rexml"
   gem "timecop"
   gem "webrat", "0.7.3", require: false
-  gem "mocha", "~> 1.1", require: false
+  gem "mocha", "~> 2.1", require: false
 end
 
 platforms :ruby do