Fix broken parser spec on Ruby 1.9.3

The behavior of `raise_error` with just the class name is strange on
Ruby 1.9.3, in that it doesn't recognize that `Haml:Error` is indeed the
same class as the actual exception.

This appears to be a bug not worth pursuing at this time, so change to
using a regex.

Change-Id: I0f17b3bc33ba08d1dc1fc45103b16127062b4e98
Reviewed-on: http://gerrit.causes.com/42556
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>