ruby: Typelib::EnumType has no convertion_to_ruby

[D-Alex]

The use case is here:
rock-core/gui-vizkit@43888b7.
Right now it must explicitly checked for EnumType.

[doudou]

An example to declare these builtin conversions can be found here:

https://github.com/orocos-toolchain/typelib/blob/master/bindings/ruby/lib/typelib/numeric_type.rb#L7

[doudou]

Follow-up, because of this, pushing a Symbol on a /std/vector fails because the conversion-from-ruby is not present