Fix version filtering and deprecation warning

The original `reject` had no effect due to matching full paths and
comparing `false` against a regular expression.

This change replaces a non-functional `reject.map` with `map.grep`
for correct filtering and resolves a deprecation warning.

Close #266

lib/document.rb

@@ -101,7 +101,7 @@ def title=(new_title)
     # Returns an array of the document version identifiers.
     def versions
       versions = Dir.glob(directory + 'v*')
-      versions.reject { |v| !v =~ /^v\d+$/ }.map { |v| File.basename v }.sort
+      versions.map { |v| File.basename v }.grep(/\Av\d+\z/).sort
     end
 
     # Returns true if the document has the specified version.