Update README with minimal ruby (3.1) and s/match/match?

[esparta]

Description

In #200 the gemspec file was changed to enforce the requirement of a minimal version of ruby 3.1, this change was not reflected on the README.md which still suggest that the gem supports ruby 2.6 and above.

Took the opportunity to also change the use of Regex#match? instead of Regex#match, this change will gain a performance improvement - it was added in ruby 2.4, specially in Pusher::Channel class. If required I can open up a new PR about that.

micro-benchmark:

# spec/benchmark.rb

require 'benchmark'
require_relative 'spec_helper'
require 'pusher'

puts Benchmark.measure {
  500_000.times do
    Pusher::Channel.new('nl', 'yo', nil)
end

before

ruby spec/benchmark.rb
  0.342884   0.000089   0.342973 (  0.344086)

after

ruby spec/benchmark.rb
  0.272801   0.000000   0.272801 (  0.273464),

CHANGELOG

• [CHANGED] Change README.md to reflect the minimal version supported
• [CHANGED] use Regex#match? instead of Regex#match to improve overall performance of the gem

Notes

I didn't add the benchmark file, I can do that if the maintainers consider it worthy