Set podfiles in Travis for caching (#1012)

Summary:
It solves the warning about caching not working but this doesn't really help us right now as builds are timing out and the cache is never filled in the first place.

Based on this: 56904692d4/lib/travis/build/script/objective_c.rb (L127)
Pull Request resolved: https://github.com/facebook/flipper/pull/1012

Reviewed By: nikoant

Differential Revision: D21042282

Pulled By: passy

fbshipit-source-id: b8e9475cd58033a8840a49b0897ac1dfffc0ddb3

.travis.yml

@@ -42,6 +42,7 @@ matrix:
         - yarn build --linux --version=$TRAVIS_BUILD_NUMBER
 
     - language: objective-c
+      podfile: iOS/Sample/Podfile
       cache:
         cocoapods: true
         directories:
@@ -69,6 +70,7 @@ matrix:
         - tail -100 ./build.log
 
     - language: objective-c
+      podfile: iOS/SampleSwift/Podfile
       cache:
         cocoapods: true
         directories:
@@ -93,6 +95,7 @@ matrix:
         - tail -100 ./build.log
 
     - language: objective-c
+      podfile: iOS/Tutorial/Podfile
       cache:
         cocoapods: true
         directories: