Improve log message consistency over Ruby versions

Using #to_s here is unnecessary, as it's interpolated into a string.
Also, using #inspect provides consistency across older Ruby versions
(Array#to_s used to be blank for an empty array, but is "[]" in later
versions).
Use Set class in Derelict::Parser::Status#vm_names

Since this returns an un-ordered Array with no duplicates, it's best to
use a Set to represent this (as it prevents duplicates, and doesn't
consider order to be meaningful).