LogLevel constant values do not comply with RFC 5424

Discussion: https://groups.google.com/forum/#!topic/php-fig/Rc5YDhNdGz4

Corresponding `Psr/Log` package change: php-fig/log#21

Origin: php-fig/log#8

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The only precedent for Errata appears to be in the PSR-2 meta document (detached from the actual spec).  PSR-3 does not have a meta document.