Cast all arguments for Kernel#exec to string.

In the FFMPEG formula the revision number is represented as an integer,
this ends up as an argument to Kernel#exec and it doesn't really like
that. The fix is to cast all arguments to a string.

Fixes Homebrew/homebrew#171.

Library/Homebrew/utils.rb

@@ -88,7 +88,7 @@ end
 def safe_system cmd, *args
   puts "#{cmd} #{args*' '}" if ARGV.verbose?
   fork do
-    exec(cmd, *args) rescue nil
+    exec(cmd, *args.map(&:to_s)) rescue nil
     exit! 1 # never gets here unless exec failed
   end
   Process.wait