Workaround for cc-test-reporter with SimpleCov 0.18

This PR fixes the following build error when using cc-test-reporter with
SimpleCov 0.18.

```consle
% #!/bin/bash -eo pipefail
  ./cc-test-reporter before-build
  rake coverage
  ./cc-test-reporter after-build --exit-code $?

(snip)

Coverage report generated for RSpec to
/home/circleci/project/coverage. 5051 / 5064 LOC (99.74%) covered.
Error: json: cannot unmarshal object into Go struct field input.coverage
of type []formatters.NullInt
```

https://circleci.com/gh/rubocop-hq/rubocop-rspec/10604

This patch is a workaround until the following issue will be resolved.
codeclimate/test-reporter#418

rubocop-rspec.gemspec

@@ -41,6 +41,9 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'rack'
   spec.add_development_dependency 'rake'
   spec.add_development_dependency 'rspec', '>= 3.4'
-  spec.add_development_dependency 'simplecov'
+  # Workaround for cc-test-reporter with SimpleCov 0.18.
+  # Stop upgrading SimpleCov until the following issue will be resolved.
+  # https://github.com/codeclimate/test-reporter/issues/418
+  spec.add_development_dependency 'simplecov', '< 0.18'
   spec.add_development_dependency 'yard'
 end