BUG Exclude framework/dev from text collection (fixes #4754)

Avoids breaking execution if PHPUnit is not installed,
since classes in framework/dev/ extend PHPUnit,
and get autoloaded when we inspect the ancestry and implementors
of every class defined on the filesystem.
Not an ideal, since it removes i18n support for dev classes,
but in practice this only impacts a single entity definition.

See discussion at https://groups.google.com/forum/?fromgroups=#!topic/silverstripe-dev/5hMXL1zeh1o