Add simplecov as a development dependency

[kdiogenes]

Without simplecov bundle exec rspec gives a bunch of errors like:

An error occurred while loading ./spec/better_errors/code_formatter_spec.rb.
Failure/Error: require 'simplecov'

LoadError:
  cannot load such file -- simplecov
# ./spec/spec_helper.rb:16:in `require'
# ./spec/spec_helper.rb:16:in `<top (required)>'
# ./spec/better_errors/code_formatter_spec.rb:1:in `require'
# ./spec/better_errors/code_formatter_spec.rb:1:in `<top (required)>'

[RobinDaugherty]

Thanks for your contribution @cerdiogenes!

Simplecov (through Coveralls) is already included in each of the Gemfiles, where require: false can be used. This is required in order to have simplecov work correctly—it needs to be required at the top of spec_helper, and must not be loaded by Bundler before that.

I made this change while merging this PR.