Specs for Ruby 1.9 API changes to Process.(g|s)et_rlimit

As of Ruby 1.9 `Process.getrlimit` and `Process.setrlimit` can be passed
a resource name in the form of a Symbol instead of a Process::RLIMIT_*
constant.

This commit includes specs for both get and set functions to verify the
correct behaviour of Rubinius' implementation.
Add symbols as resources to Process.(g|s)et_rlimit

As of Ruby 1.9 you can pass a symbol as the name of the resource you're
operating on instead of using the constant form.  For example:

    Process.getrlimit(:CORE)
    => [0, 9223372036854775807]

whereas traditionally one would have used:

    Process.getrlimit(Process::RLIMIT_CORE)
    => [0, 9223372036854775807]

This commit adds behaviour to Process.get_rlimit and Process.set_rlimit
to infer and look up a constant name from a resource name supplied as a
Symbol.