Make coveralls a runtime dependency

When executing the instructions in the README, I got a load error:

```
/Users/jh/.rubies/ruby-2.1.5/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require': cannot load such file -- coveralls (LoadError)
	from /Users/jh/.rubies/ruby-2.1.5/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /Users/jh/Code/Ruby/pry-test/test/test_helper.rb:1:in `<top (required)>'
	from /Users/jh/Code/Ruby/pry-test/test/cpu_latency_test.rb:2:in `require_relative'
	from /Users/jh/Code/Ruby/pry-test/test/cpu_latency_test.rb:2:in `<top (required)>'
	from /Users/jh/.rubies/ruby-2.1.5/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /Users/jh/.rubies/ruby-2.1.5/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from ./bin/pry-test:59:in `block in <main>'
	from ./bin/pry-test:59:in `each'
	from ./bin/pry-test:59:in `<main>'
```

By making Coveralls a runtime dependency, this won't be an issue. Let me know if this causes other issues I'm not aware of.