Refresh gemspec

Just cleanup, no breaking changes like an explicit license or required
Ruby version.

Author: mostlyobvious

protobuf_nested_struct.gemspec

@@ -1,32 +1,31 @@
-lib = File.expand_path("../lib", __FILE__)
-$LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
-require "protobuf_nested_struct/version"
+# frozen_string_literal: true
+
+require_relative "lib/protobuf_nested_struct/version"
 
 Gem::Specification.new do |spec|
   spec.name = "protobuf_nested_struct"
   spec.version = ProtobufNestedStruct::VERSION
   spec.authors = ["Robert Pankowecki", "Arkency"]
   spec.email = ["[email protected]"]
-
   spec.summary =
     "Serialize primitives and deep structures (array, hash) to protobuf"
-  spec.description =
-    "Serialize primitives and deep structures (array, hash) to protobuf"
+  spec.description = <<~EOD
+    Serialize primitives and deep structures (array, hash) to protobuf
+  EOD
   spec.homepage = "https://github.com/arkency/protobuf-nested-struct"
+  spec.files = Dir["lib/**/*"]
+  spec.require_paths = %w[lib]
+  spec.extra_rdoc_files = %w[README.md]
+
   spec.metadata = {
     "homepage_uri" => "https://github.com/arkency/protobuf-nested-struct",
     "changelog_uri" =>
       "https://github.com/arkency/protobuf-nested-struct/releases",
     "source_code_uri" => "https://github.com/arkency/protobuf-nested-struct",
     "bug_tracker_uri" =>
-      "https://github.com/arkency/protobuf-nested-struct/issues"
+      "https://github.com/arkency/protobuf-nested-struct/issues",
+    "rubygems_mfa_required" => "true"
   }
 
-  spec.files =
-    `git ls-files -z`.split("\x0")
-      .reject { |f| f.match(%r{^(test|spec|features)/}) }
-  spec.bindir = "exe"
-  spec.executables = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
-  spec.require_paths = ["lib"]
   spec.add_dependency "google-protobuf", "< 4"
 end