Require rspec/matchers before require enumerate/integrations/rspec.

Using Rails 4.0.3, Ruby 2.1 with `rails/spring`.

When `if defined?(::RSpec)` is used, and specs are ran with either `spring rake` or `spring rspec` it fails to load the enumerize matcher properly.

    NoMethodError:
       undefined method `enumerize' for #<RSpec::Core::ExampleGroup::Nested_2::Nested_1:0x000001014521b0>

As described in https://github.com/rails/spring/issues/209, the solution is to `require rspec/matchers` before the enumerize matcher.