Change gemspec to allow installation on Ruby 3.0 and all future versions

This change allows the gem to be installed on Ruby 3.0 and all future
versions. It also loads the gem version directly using a relative path
and removes encoding comment as UTF-8 is a default encoding in Ruby 2.0.

java-properties.gemspec

@@ -1,7 +1,6 @@
-# coding: utf-8
-lib = File.expand_path('../lib', __FILE__)
-$LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
-require 'java-properties/version'
+# frozen_string_literal: true
+
+require_relative 'lib/java-properties/version'
 
 Gem::Specification.new do |spec|
   spec.name          = 'java-properties'
@@ -19,7 +18,7 @@ Gem::Specification.new do |spec|
   spec.test_files = Dir.glob('spec/fixtures/**/*.properties')
 
   spec.required_rubygems_version = '>= 1.3.5'
-  spec.required_ruby_version = '~> 2.0'
+  spec.required_ruby_version = '>= 2.0.0'
 
   spec.add_development_dependency 'rake', '~> 13.0'
   spec.add_development_dependency 'inch', '~> 0.8'