Provide own native gems for Ruby 2.6 and Protobuf.

This allows mutation/testing on Ruby 2.6.

Related:
https://twitter.com/pawelpacana/status/1080576829872844801
protocolbuffers/protobuf#5161

Private source to be removed once there's an official update on
rubygems.org.

Building https://github.com/protocolbuffers/protobuf:
1. compile C++ version to have src/proto ready for use
2. on macos: rake build clobber_package gem gem:native
3. on linux: rake build clobber_package gem gem:native as well, but make
   sure to update rake-compiler* dependencies to newest ones, update
   RUBY_CC_VERSION to include 2.6.0 and disable non-linux from ext.cross_platform

lib/protobuf_helper.rb

@@ -9,7 +9,7 @@ def require_protobuf_dependencies
   end
 
   def unsupported_ruby_version
-    truffleruby || jruby || ruby_2_6_0
+    truffleruby || jruby
   end
 
   def truffleruby
@@ -19,8 +19,4 @@ def truffleruby
   def jruby
     RUBY_PLATFORM == "java"
   end
-
-  def ruby_2_6_0
-    RUBY_VERSION == "2.6.0"
-  end
 end

rails_event_store/Gemfile

@@ -12,6 +12,6 @@ gem 'protobuf_nested_struct'
 gem 'sidekiq'
 gem 'sqlite3'
 gem 'rack-test'
-gem 'google-protobuf', '~> 3.5.1.2'
+gem 'google-protobuf', '= 3.6.1', source: 'https://gem.fury.io/pawelpacana/'
 gem 'pry'
 gem 'rails', ENV['RAILS_VERSION']

ruby_event_store/Gemfile

@@ -7,5 +7,5 @@ gem 'protobuf_nested_struct'
 gem 'parser'
 gem 'unparser'
 gem 'astrolabe'
-gem 'google-protobuf', '~> 3.5.1.2'
-gem 'activesupport', '~> 5.0'
+gem 'google-protobuf', '= 3.6.1', source: 'https://gem.fury.io/pawelpacana/'
+gem 'activesupport', '~> 5.0'