DefaultEnvironment should honor the order of environment names

[thiagolocatelli]

DefaultEnvironment, line 127 is using Set<String> specifiedNames to store the environment names.

By using Set<String> the order of the environment names provided by the developer is not being honored. List<String> should be used instead.

(I am implementing a discovery client for Spring Cloud config server and the order provided matters to the server, issue #501 - link).

[jameskleeh]

@thiagolocatelli To prevent a breaking change, I simply changed the implementation to use a LinkedHashSet. That way order and uniqueness is preserved.