Optimize `RSpec::CallerFilter`.

- Use `caller_locations` rather than `caller`. It's apparently
  much faster. I suspect that it's due to the fact that `caller`
  has to due some extra calculation for each stack frame in order
  to combine them into a full line description string, where as
  `caller_locations` returns an object with the separate stack
  frame attributes and does the full line description calculation
  when you call `to_s`.
- Immediately return when we have the line, rather than updating `i`
  and `increment` for now reason.
Merge pull request #140 from rspec/optimize-caller-filter

Optimize `RSpec::CallerFilter`.