Previously, configuration properties binding would only see changes to
the environment if there was no PropertySourcesPlaceholderConfigurer
in the context. This happened because
PropertySourcesPlaceholderConfigurer wrapped the Environment in a
PropertySource, effectively hiding it from the change tracking
performed by SpringConfigurationPropertySources.

This commit updates ConfigurationPropertiesBindingPostProcessor so
that it ignores the environment property source contained by
PropertySourcesPlaceholderConfigurer and uses a composite of
the PropertySourcesPlaceholderConfigurer's other property sources and
the Environment's PropertySources instead.