Avoid keeping track of two lists of nodes during parsing.
Use super rather than render_all in single block render classes.
Merge pull request #445 from Shopify/prefer-super-over-render-all

Use super rather than render_all in single block render classes.
Merge pull request #444 from Shopify/remove-block-children

Avoid keeping track of two lists of nodes during parsing.
Remove unused Block#end_tag method.

Although the method is called, it is defined with an empty body and not
overridden to do anything else.