Fix Bundler/Capistrano errors by not requiring rake/file_list in gemspec (using Dir.glob instead)

Use Dir[] instead of Rake FileList[].

Before I made this change, I got errors like

 ** [out :: example.com] There was a LoadError while evaluating metar-parser.gemspec:
 ** [out :: example.com] no such file to load -- rake/file_list from
 ** [out :: example.com] /var/www/example.com/shared/bundle/ruby/1.8/bundler/gems/metar-parser-b3a058682c38/metar-parser.gemspec:4

trying to deploy  with capistrano and bundler.

@see "Using .gemspecs as Intended" 
http://yehudakatz.com/2010/04/02/using-gemspecs-as-intended/
"…Do not, however, use other libraries or dependencies…"