Fix the podlint error (Instagram#1428)

Summary:
Pull Request resolved: Instagram#1428

We kept getting an error as the following:

```
    - ERROR | [OSX] unknown: Encountered an unknown error (The platform of the target `App` (macOS 10.11) is not compatible with `IGListSwiftKit (4.1.0)`, which does not support `macOS`.) during validation.
```

Looks like IGListSwiftKit did not support macos, wondering why this is the case here.

Potentially this error was introduced in this PR: Instagram#1388
cc natestedman who might know the context here

The solution here is to lint the specs separately in travis, so that for IGListSwiftKit we only lint against ios and tvos. And the other podspecs will still lint against on all platform, including the macos.

Reviewed By: natestedman

Differential Revision: D20036436

fbshipit-source-id: 67c1578779a20435af0c43428c40a38ad18265b3

.travis.yml

@@ -58,8 +58,11 @@ jobs:
  name: Cocoapods Lint
  # We need to pass all of the podspecs here so that Cocoapods uses the local version,
  # rather than searching the specs repository for the other specs in the repo.
- script: bundle exec pod lib lint --allow-warnings "--include-podspecs=*.podspec"
-
+ script: 
+ - bundle exec pod lib lint IGListDiffKit.podspec --allow-warnings
+ - bundle exec pod lib lint IGListKit.podspec --allow-warnings "--include-podspecs=IGListDiffKit.podspec"
+ - bundle exec pod lib lint IGListSwiftKit.podspec --allow-warnings "--include-podspecs=*.podspec"
+
  # Build example projects
  - &build-examples
  stage: build examples