Add location for attribute

[ksss]

I noticed that location is not set for the attribute method. Is this intentional?

I actually have some problems with the rbs method command, such as not being able to find the location where the attribute method is defined.

I think it is better to have attribute location than not to have it, so I fixed it.

---

Before

$ bundle exec rbs -r rbs method --singleton RBS::Test suffix
::RBS::Test.suffix
  defined_in: ::RBS::Test
  implementation: ::RBS::Test
  accessibility: public
  types:
      () -> ::String   at

After

$ bundle exec rbs -r rbs method --singleton RBS::Test suffix
::RBS::Test.suffix
  defined_in: ::RBS::Test
  implementation: ::RBS::Test
  accessibility: public
  types:
      () -> ::String   at /Users/ksss/src/github.com/ksss/rbs/sig/test.rbs:76:4...76:37

[soutaro]

Thanks!