[Fastlane] Specify screenshot files only, rather than whole folder (in case the user has frameit config)

The whole screenshots folder shouldn't be ignore since there could be frameit configuration in it (Framefile.json, fonts, background.jpg, *.strings, etc...). It'd be better to just ignore the generated PNG screenshot files.

Author: juliancadi

Objective-C.gitignore

@@ -52,7 +52,7 @@ Carthage/Build
 
 fastlane/report.xml
 fastlane/Preview.html
-fastlane/screenshots
+fastlane/screenshots/**/*.png
 fastlane/test_output
 
 # Code Injection