Print error to STDERR in script '📦 Check Pods Manifest.lock'

This ensures that scripts like xcpretty can differentiate default output
and error output.

lib/cocoapods/installer/user_project_integrator/target_integrator.rb

@@ -176,9 +176,8 @@ module Pod
             phase.shell_script = <<-SH.strip_heredoc
               diff "${PODS_ROOT}/../Podfile.lock" "${PODS_ROOT}/Manifest.lock" > /dev/null
               if [ $? != 0 ] ; then
-                  cat << EOM
-              error: The sandbox is not in sync with the Podfile.lock. Run 'pod install' or update your CocoaPods installation.
-              EOM
+                  # print error to STDERR
+                  echo "error: The sandbox is not in sync with the Podfile.lock. Run 'pod install' or update your CocoaPods installation." >&2
                   exit 1
               fi
             SH

spec/unit/installer/user_project_integrator/target_integrator_spec.rb

@@ -97,9 +97,8 @@ module Pod
           phase.shell_script.should == <<-EOS.strip_heredoc
           diff "${PODS_ROOT}/../Podfile.lock" "${PODS_ROOT}/Manifest.lock" > /dev/null
           if [ $? != 0 ] ; then
-              cat << EOM
-          error: The sandbox is not in sync with the Podfile.lock. Run 'pod install' or update your CocoaPods installation.
-          EOM
+              # print error to STDERR
+              echo "error: The sandbox is not in sync with the Podfile.lock. Run 'pod install' or update your CocoaPods installation." >&2
               exit 1
           fi
           EOS