Add support for local and instance vars to SimpleCov DSL

This commit replaces the naked instance_exec call with a
call to Docile#dsl_eval, which abstracts away the complexities
of referring to outside variables in Ruby DSLs.
Merge pull request #238 from ms-tg/support-instance-and-local-vars-in-dsl

Add support for local and instance vars to SimpleCov DSL