Add opt-in code coverage reporting

Run tests with, e.g., `COVERAGE=t bundle exec rspec PATH` to generate a
report

1 files changed, 1 insertions, 0 deletions

diff --git a/Gemfile b/Gemfile
index 306a0b18b0..4f505f6f38 100644
--- a/Gemfile
+++ b/Gemfile
@@ -9,6 +9,7 @@ group(:docgen) do
 end
 
 group(:development, :test) do
+  gem "simplecov"
   gem 'rack', "~> 1.5.1"
 
   gem 'ruby-shadow', :platforms => :ruby unless RUBY_PLATFORM.downcase.match(/(darwin|freebsd)/)