Support rspec-2 and rspec-3

This updates guard-rspec to work with rspec >= 2.14, < 4

This allows guard-rspec users to choose when to upgrade to rspec-3 without having to also update guard-rspec at the same time (assuming they've at least upgraded to a version of guard-rspec that includes this PR).

It is also mostly compatible with guard-rspec developers being able to choose rspec-2.x or rspec-3.x. The only thing missing is that if you are using rspec-2.x and want to use `:focus` or `:focused`, it won't work without `:focus => true`. If you prefer, I can add a guard for that functionality e.g.

```ruby
config.treat_symbols_as_metadata_keys_with_true_values = true if RSpec::Version::STRING.to_i < 3
```