Replace [[ with [ in script '📦 Check Pods Manifest.lock'

The script is integrated using /bin/sh but [[ is not defined in generic
POSIX shell. Confirm shellcheck error:

if [[ $? != 0 ]] ; then
   ^-- SC2039: In POSIX sh, [[ ]] is undefined.

This works right now, because on common OS X systems, /bin/sh is Bash.
But this might change in the future.

lib/cocoapods/installer/user_project_integrator/target_integrator.rb

@@ -175,7 +175,7 @@ module Pod
             native_target.build_phases.unshift(phase).uniq! unless native_target.build_phases.first == phase
             phase.shell_script = <<-SH.strip_heredoc
               diff "${PODS_ROOT}/../Podfile.lock" "${PODS_ROOT}/Manifest.lock" > /dev/null
-              if [[ $? != 0 ]] ; then
+              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

spec/unit/installer/user_project_integrator/target_integrator_spec.rb

@@ -96,7 +96,7 @@ module Pod
           phase = target.shell_script_build_phases.find { |bp| bp.name == phase_name }
           phase.shell_script.should == <<-EOS.strip_heredoc
           diff "${PODS_ROOT}/../Podfile.lock" "${PODS_ROOT}/Manifest.lock" > /dev/null
-          if [[ $? != 0 ]] ; then
+          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