use custom travis script to avoid code sign in test

Author: morganchen12

.travis.yml

@@ -3,3 +3,13 @@ language: objective-c
 xcode_workspace: FirebaseUI.xcworkspace
 xcode_scheme: FirebaseUITests
 
+script: xcodebuild -workspace FirebaseUI.xcworkspace
+  -scheme FirebaseUITests
+  -sdk iphonesimulator
+  -destination 'platform=iOS Simulator,name=iPhone 6s'
+  build 
+  test
+  ONLY_ACTIVE_ARCH=NO
+  CODE_SIGN_IDENTITY=""
+  CODE_SIGNING_REQUIRED=NO
+