Fix mangle filter

Right now the mangler will miss libraries that have the main pod name within their name. For example, if you're mangling for `A.podspec` which depends on `AFNetworking`, the `AFNetworking` lib will not be mangled because it contains the letter `A`.
Can this not be `/^libPods-#{pod_name}.a$/`, or even better, not use regex at all now?

```ruby
file == "libPods-#{pod_name}.a"
```