Specification#add_runtime_dependency accepts instances of Dependency

Based on the unless block, it looks like I should be able to pass a `Gem::Dependency`, which has a name and an ivar `@version_requirements`, but no method for it.

It got [deprecated](41bc3129d3976c6ac415241181b902052878a9bd) and should now be `#requirements`

Test Failure without this patch:

```diff
-Gem::Dependency.new("existing_dep", Gem::Requirement.new(["> 1"]), :runtime)
+Gem::Dependency.new("existing_dep (> 1)", Gem::Requirement.new([">= 0"]), :runtime)
```