Fix bundler vendoring/caching (sinatra#2144)

When using Bundler's bundle package to vendor/cache gems,
sinatra-contrib and rack-protection gemspecs failed with:
No such file or directory @ rb_sysopen - /path/to/vendor/cache/VERSION
Both gemspecs read the version from a parent VERSION file using
../VERSION. When Bundler copies gems to vendor/cache, the parent
directory structure doesn't exist, so ../VERSION points to a
non-existent file.

Author: MuhammadIbtisam

rack-protection/rack-protection.gemspec

@@ -1,11 +1,11 @@
 # frozen_string_literal: true
 
-version = File.read(File.expand_path('../VERSION', __dir__)).strip
+require_relative 'lib/rack/protection/version'
 
 Gem::Specification.new do |s|
   # general infos
   s.name        = 'rack-protection'
-  s.version     = version
+  s.version     = Rack::Protection::VERSION
   s.description = 'Protect against typical web attacks, works with all Rack apps, including Rails'
   s.homepage    = 'https://sinatrarb.com/protection/'
   s.summary     = "#{s.description}."

sinatra-contrib/sinatra-contrib.gemspec

@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
-version = File.read(File.expand_path('../VERSION', __dir__)).strip
+require_relative 'lib/sinatra/contrib/version'
+
+version = Sinatra::Contrib::VERSION
 
 Gem::Specification.new do |s|
   s.name        = 'sinatra-contrib'