Change rest_server check for spyc library to a runtime requirement not an install requirement

The rest_server module can not be installed via an install profile in some cases. Because of the fact that hook_requirements for the module is run before drupal itself or any other module has been installed trying to include rest_server in an installation profile causes problems. It attemtps to use the module_include() function which depends on the {system} table which doesn't exist yet.

I think the appropriate fix here is to check for the existence of the spyc library at runtime instead of install time. The rest_server module works just fine without the spyc library (you just can't use YAML) so it's not really a requirement to be able to install the module. Changing it to $phase == 'runtime' also eliminates the issue of not being able to include rest_server in an install profile.

Patch to follow shortly.