Convert license to array of identifiers

RubyGems currently complain during gem build:

~~~
WARNING:  license value 'MIT AND (BSD-2-Clause OR GPL-2.0-or-later)' is invalid.  Use a license identifier from
http://spdx.org/licenses or 'Nonstandard' for a nonstandard license.
Did you mean 'AGPL-1.0-or-later', 'AGPL-3.0-or-later', 'BSD-2-Clause-Patent', 'GPL-2.0-or-later', 'LGPL-2.0-or-later'?
~~~

Nothing else then plain list of license identifers is supported ATM.

Relates #309

nio4r.gemspec

@@ -6,7 +6,7 @@ Gem::Specification.new do |spec|
   spec.authors       = ["Tony Arcieri"]
   spec.email         = ["bascule@gmail.com"]
   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", "GPL-2.0-or-later"]
   spec.summary       = "New IO for Ruby"
   spec.description   = <<-DESCRIPTION.strip.gsub(/\s+/, " ")
      Cross-platform asynchronous I/O primitives for scalable network clients