Fix for issue `Fatal error:  Call to undefined method stdClass::getEnd() in Prismic/Fragment/StructuredText.php on line 378`

If there's some reason why there shouldn't be spans in <pre>, then the spans should probably be discarded with `return new PreformattedBlock($json->text, array(), $label);`
Merge pull request #70 from gsteel/fix-spans-in-preformatted-blocks

Fix for issue `Fatal error:  Call to undefined method stdClass::getEnd()...
Support for h5, h6 blocks