[Installer] Use downcase to validate configurations

Gemfile.lock

@@ -7,7 +7,7 @@ GIT
 
 GIT
   remote: https://github.com/CocoaPods/Core.git
-  revision: 509edfe14aa7ef1fb29a50c1c9935309d614c7c4
+  revision: 01a86d5160700165a7e75b0c98c00d5d5ea48a4e
   branch: lookback-pods-by-config-2
   specs:
     cocoapods-core (0.33.1)
@@ -18,7 +18,7 @@ GIT
 
 GIT
   remote: https://github.com/CocoaPods/Xcodeproj.git
-  revision: f7e2a798464b7f4fce49379a748490a8ba83549a
+  revision: 96d04e9f2d96b848bb36d5fe214843f8cfac2664
   branch: master
   specs:
     xcodeproj (0.18.0)

lib/cocoapods/installer.rb

@@ -117,7 +117,6 @@ module Pod
         install_file_references
         install_libraries
         set_target_dependencies
-        # note: linking with pod libraries is now done by aggregate_xcconfig's OTHER_LDFLAGS
         run_post_install_hooks
         write_pod_project
         write_lockfiles
@@ -185,7 +184,7 @@ module Pod
     #
     def validate_build_configurations
       whitelisted_configs = pod_targets.map do |target|
-        target.target_definition.all_whitelisted_configurations
+        target.target_definition.all_whitelisted_configurations.map(&:downcase)
       end.flatten.uniq
       all_user_configurations = analysis_result.all_user_build_configurations.keys.map(&:downcase)