Maintain a version dir map in box_collection.

This is required since Gem::Version will mangle pre-release version
numbers.
To get around this, we keep a mapping of
`Gem::Version.to_s => versiondir.`

This allows us to properly sort by version when picking a box while
still returning pristine versiondirs when required.

lib/vagrant/box_collection.rb

@@ -272,21 +272,30 @@ module Vagrant
           return nil
         end
 
+        # Keep a mapping of Gem::Version mangled versions => directories.
+        # ie. 0.1.0.pre.alpha.2 => 0.1.0-alpha.2
+        # This is so we can sort version numbers properly here, but still
+        # refer to the real directory names in path checks below and pass an
+        # unmangled version string to Box.new
+        version_dir_map = {}
+
         versions = box_directory.children(true).map do |versiondir|
           next if !versiondir.directory?
           next if versiondir.basename.to_s.start_with?(".")
 
           version = Gem::Version.new(versiondir.basename.to_s)
+          version_dir_map[version.to_s] = versiondir.basename.to_s
+          version
         end.compact
 
         # Traverse through versions with the latest version first
         versions.sort.reverse.each do |v|
-          if !requirements.all? { |r| r.satisfied_by?(Gem::Version.new(v)) }
+          if !requirements.all? { |r| r.satisfied_by?(v) }
             # Unsatisfied version requirements
             next
           end
 
-          versiondir = box_directory.join(v.to_s)
+          versiondir = box_directory.join(version_dir_map[v.to_s])
           providers.each do |provider|
             provider_dir = versiondir.join(provider.to_s)
             next if !provider_dir.directory?
@@ -303,7 +312,7 @@ module Vagrant
             end
 
             return Box.new(
-              name, provider, v, provider_dir,
+              name, provider, version_dir_map[v.to_s], provider_dir,
               metadata_url: metadata_url,
             )
           end