Fix model with no updated_at or deleted_at columns

[Roxayl]

Laravel allows overriding the column name used for timestamps (by default updated_at/deleted_at), or disabling timestamps completely by setting the UPDATED_AT and DELETED_AT constants to null (docs). With strict enough error reporting settings, whenever one of those columns is disabled (e.g. set to null), an ErrorException is thrown:

versionable/src/Mpociot/Versionable/VersionableTrait.php

Line 273 in 9af7957

return ( count(array_diff_key($this->versionableDirtyData, array_flip($removeableKeys))) > 0 );

ErrorException: array_flip(): Can only flip string and integer values, entry skipped

The pull request adds checks to ensure the column names for timestamps is valid/not null.

[nonoesp]

Thanks @Roxayl!