Fix cocoapods issues

[ekharkunov]

There were two issues that I found during cocoapods integration testing:

1. When command pod spec cat --regex ... produce some additional output like

Ignoring ffi-1.15.4 because its extensions are not built. Try: gem pristine ffi --version 1.15.4

build failed with exception.
2. If pod spec has vendored_frameworks section under platform section - it ignores and produces linker error later. For example, UnityAds pod has following spec

{
  "name": "UnityAds",
  "version": "4.2.1",
  "license": {
    "type": "Unity License",
    "file": "LICENSE"
  },
  "authors": {
    "UnityAds": "itunes@unity3d.com"
  },
  "homepage": "https://unity3d.com/services/ads",
  "summary": "Monetize your entire player base and reach new audiences with video ads.",
  "platforms": {
    "ios": "9.0"
  },
  "source": {
    "http": "https://github.com/Unity-Technologies/unity-ads-ios/releases/download/4.2.1/UnityAds.zip"
  },
  "ios": {
    "vendored_frameworks": "UnityAds.xcframework"
  }
}

[britzl]

Looks good! Thanks.