FIX: pick the correct version of libv8 in extconf

Previously on downgrade we could pick the wrong version.

This will correct the issue going forward, but we have no way of fixing
history

Fixes: #171

CHANGELOG

@@ -1,4 +1,8 @@
-- 23-07-202
+- Unreleased
+
+  - FIX: on downgrade mkmf was picking the wrong version of libv8, this fix will correct future issues
+
+- 23-07-2020
 
 - 0.3.1
 

ext/mini_racer_extension/extconf.rb

@@ -1,4 +1,6 @@
 require 'mkmf'
+require_relative '../../lib/mini_racer/version'
+gem 'libv8', MiniRacer::LIBV8_VERSION
 require 'libv8'
 
 IS_DARWIN = RUBY_PLATFORM =~ /darwin/

lib/mini_racer/version.rb

@@ -2,4 +2,5 @@
 
 module MiniRacer
   VERSION = "0.3.1"
+  LIBV8_VERSION = "~> 8.4.255"
 end

mini_racer.gemspec

@@ -32,7 +32,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "rake-compiler"
   spec.add_development_dependency "m"
 
-  spec.add_dependency 'libv8', '~> 8.4.255'
+  spec.add_dependency 'libv8', MiniRacer::LIBV8_VERSION
   spec.require_paths = ["lib", "ext"]
 
   spec.extensions = ["ext/mini_racer_extension/extconf.rb"]