Bump minimum version of webmock to 3.8.0 (#897)

Bumps the minimum version of webmock to 3.8.0 which contains a fix for a
Ruby 2.7 warning. This last change makes the whole project completely
warning-free!

Co-authored-by: Brandur <brandur@brandur.org>

Gemfile

@@ -11,7 +11,10 @@ group :development do
  gem "rake"
  gem "shoulda-context"
  gem "test-unit"
- gem "webmock"
+
+ # Version doesn't matter that much, but this one contains some fixes for Ruby
+ # 2.7 warnings that add noise to the test suite.
+ gem "webmock", ">= 3.8.0"
 
  # Rubocop changes pretty quickly: new cops get added and old cops change
  # names or go into new namespaces. This is a library and we don't have