Convert license to array of identifiers (#312)

- nio4r code is licensed under the MIT license.
- libev code is licensed under the BSD-2-Clause license.

nio4r.gemspec

@@ -6,7 +6,7 @@ Gem::Specification.new do |spec|
   spec.authors       = ["Tony Arcieri"]
   spec.email         = ["[email protected]"]
   spec.homepage      = "https://github.com/socketry/nio4r"
-  spec.license       = "MIT AND (BSD-2-Clause OR GPL-2.0-or-later)"
+  spec.licenses      = ["MIT", "BSD-2-Clause"]
   spec.summary       = "New IO for Ruby"
   spec.description   = <<-DESCRIPTION.strip.gsub(/\s+/, " ")
      Cross-platform asynchronous I/O primitives for scalable network clients