RSpec3 travis test suite

I added another gemfile set to test rspec 3 directly on travis (no code changes BTW). The travis logs show the same issues I ran into when I tested rspec3 environment manually. Unfortunately I was unable to deduce how to correct these issues. 

```
Failures:
  1) Mutant::Expression::Namespace::Recursive#matcher 
     Failure/Error: Unable to find matching line from backtrace
     NoMethodError:
       undefined method `matchable_scopes' for #<Mutant::Cache>
     # ./lib/mutant/matcher/namespace.rb:23:in `each'
  2) Mutant::Expression::Namespace::Exact#matcher 
     Failure/Error: Unable to find matching line from backtrace
     NoMethodError:
       undefined method `cache' for #<Mutant::Cache>
     # ./lib/mutant/matcher/method.rb:77:in `ast'
     # ./lib/mutant/matcher/method.rb:138:in `matched_node'
     # ./lib/mutant/matcher/method.rb:121:in `subject'
     # ./lib/mutant/matcher/method.rb:25:in `each'
     # ./lib/mutant/matcher/methods.rb:58:in `to_a'
     # ./lib/mutant/matcher/methods.rb:58:in `each'
     # ./lib/mutant/matcher/methods.rb:58:in `flat_map'
     # ./lib/mutant/matcher/methods.rb:58:in `subjects'
     # ./lib/mutant/matcher/methods.rb:20:in `each'
     # ./lib/mutant/matcher/scope.rb:37:in `block in each'
     # ./lib/mutant/matcher/scope.rb:36:in `each'
     # ./lib/mutant/matcher/scope.rb:36:in `each'
  3) Mutant::Expression::Method#matcher with a singleton method 
     Failure/Error: Unable to find matching line from backtrace
     NoMethodError:
       undefined method `cache' for #<Mutant::Cache>
     # ./lib/mutant/matcher/method.rb:77:in `ast'
     # ./lib/mutant/matcher/method.rb:138:in `matched_node'
     # ./lib/mutant/matcher/method.rb:121:in `subject'
     # ./lib/mutant/matcher/method.rb:25:in `each'
  4) Mutant::Expression::Methods#matcher with a singleton method 
     Failure/Error: Unable to find matching line from backtrace
     NoMethodError:
       undefined method `cache' for #<Mutant::Cache>
     # ./lib/mutant/matcher/method.rb:77:in `ast'
     # ./lib/mutant/matcher/method.rb:138:in `matched_node'
     # ./lib/mutant/matcher/method.rb:121:in `subject'
     # ./lib/mutant/matcher/method.rb:25:in `each'
     # ./lib/mutant/matcher/methods.rb:58:in `to_a'
     # ./lib/mutant/matcher/methods.rb:58:in `each'
     # ./lib/mutant/matcher/methods.rb:58:in `flat_map'
     # ./lib/mutant/matcher/methods.rb:58:in `subjects'
     # ./lib/mutant/matcher/methods.rb:20:in `each'
  5) Mutant::Expression::Methods#matcher with an instance method 
     Failure/Error: Unable to find matching line from backtrace
     NoMethodError:
       undefined method `cache' for #<Mutant::Cache>
     # ./lib/mutant/matcher/method.rb:77:in `ast'
     # ./lib/mutant/matcher/method.rb:138:in `matched_node'
     # ./lib/mutant/matcher/method.rb:121:in `subject'
     # ./lib/mutant/matcher/method.rb:25:in `each'
     # ./lib/mutant/matcher/methods.rb:58:in `to_a'
     # ./lib/mutant/matcher/methods.rb:58:in `each'
     # ./lib/mutant/matcher/methods.rb:58:in `flat_map'
     # ./lib/mutant/matcher/methods.rb:58:in `subjects'
     # ./lib/mutant/matcher/methods.rb:20:in `each'
Finished in 46.07 seconds (files took 3.44 seconds to load)
544 examples, 5 failures, 6 pending
Failed examples:
rspec ./spec/unit/mutant/expression/namespace/recursive_spec.rb:11 # Mutant::Expression::Namespace::Recursive#matcher 
rspec ./spec/unit/mutant/expression/namespace/flat_spec.rb:12 # Mutant::Expression::Namespace::Exact#matcher 
rspec ./spec/unit/mutant/expression/method_spec.rb:45 # Mutant::Expression::Method#matcher with a singleton method 
rspec ./spec/unit/mutant/expression/methods_spec.rb:40 # Mutant::Expression::Methods#matcher with a singleton method 
rspec ./spec/unit/mutant/expression/methods_spec.rb:34 # Mutant::Expression::Methods#matcher with an instance method 
```