Install simplecov, require a minimum coverage

If someone submits a PR and doesn't write tests, having a minimum
coverage could help. CI should fail, because simplecov will exit with
a non-zero status. For example:

```
Line coverage (92.48%) is below the expected minimum coverage (93.00%).
SimpleCov failed with exit 2 due to a coverage related error
```

paper_trail.gemspec

@@ -73,6 +73,7 @@ has been destroyed.
   s.add_development_dependency "rubocop-packaging", "~> 0.5.1"
   s.add_development_dependency "rubocop-performance", "~> 1.11.3"
   s.add_development_dependency "rubocop-rspec", "~> 2.4.0"
+  s.add_development_dependency "simplecov", ">= 0.21", "< 0.22"
 
   # ## Database Adapters
   #

spec/spec_helper.rb

@@ -3,6 +3,12 @@
 ENV["RAILS_ENV"] ||= "test"
 ENV["DB"] ||= "sqlite"
 
+require "simplecov"
+SimpleCov.start do
+  add_filter "spec"
+end
+SimpleCov.minimum_coverage 92.4
+
 require "byebug"
 require_relative "support/pt_arel_helpers"