Updated Gemfile.
Updated Rubinius::ToolSets constant.
Removed String#to_ast specs.

The String#to_ast method should be accessed through the parser rather
than polluting the String namespace.
Set up C-API rb_integer_pack specs.
Basic specs for rb_integer_pack.
Added basic rb_integer_pack.
More rb_integer_pack specs.
Fixed rb_integer_pack edge cases.