Bump virtus dependency to ~> 0.5

This makes it possible to have dm-core coexist
with veritas and friends.

virtus 0.5 hasn't yet been released so virtus is
pinned to github in the Gemfile for now.

Author: snusnu

Gemfile

@@ -6,6 +6,8 @@ source :rubygems
 
 gemspec
 
+gem 'virtus', '~> 0.5', :github => 'solnic/virtus'
+
 SOURCE         = ENV.fetch('SOURCE', :git).to_sym
 REPO_POSTFIX   = SOURCE == :path ? ''                                : '.git'
 DATAMAPPER     = SOURCE == :path ? Pathname(__FILE__).dirname.parent : 'http://github.com/datamapper'

dm-core.gemspec

@@ -18,7 +18,7 @@ Gem::Specification.new do |gem|
   gem.version       = DataMapper::VERSION
 
   gem.add_runtime_dependency('addressable', '~> 2.2.6')
-  gem.add_runtime_dependency('virtus',      '~> 0.4.1')
+  gem.add_runtime_dependency('virtus',      '~> 0.5')
 
   gem.add_development_dependency('rake',  '~> 0.9.2')
   gem.add_development_dependency('rspec', '~> 1.3.2')