Workaround for a build error with Prism 1.3.0

[koic]

The following test passed with Prism 1.2.0, but started failing with Prism 1.3.0.

$ bundle exec rake prism_test
(snip)

==> Failures

  1) Failure:
MultipleAssertionsTest#test_assignments_with_numblocks_are_counted_correctly [test/rubocop/cop/minitest/multiple_assertions_test.rb:244]:
--- expected
+++ actual
@@ -1,6 +1,5 @@
 "class FooTest < ActiveSupport::TestCase
   test \"#render errors include stack traces\" do
-  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Test case has too many assertions [9/1].
     err = assert_raises React::ServerRendering::PrerenderError do
       assert_equal _1, 1

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.