Update utf-8 validity tests to account for liquid checking encoding

[zachmccormick]

Shopify/liquid#1774 introduced a pre-parsing check for valid UTF-8 encoding, and Shopify/liquid#1776 changed the raised exception to Liquid::TemplateEncodingError. This updates the failing test to look for the proper exception and message.

It may also be a valid outcome to remove this test entirely since this behavior is really part of https://github.com/Shopify/liquid and not the c extension

[ggmichaelgo]

Hello @zachmccormick !
As you described on your PR, we could just remove test_invalid_utf8_sequence test.
Since the Liquid no longer allows templates with invalid UTF-8 byte sequence, this test is now obsolete.

[ggmichaelgo]

Closing this PR since #220 has been merged!