Don't use raw numbers for Entry compression types.

Constants for Store and Deflate are already available, so use them. It
might be sensible to remove these local versions, but they do have their
uses as a shortened form.

lib/zip/entry.rb

@@ -1,8 +1,9 @@
 require 'pathname'
 module Zip
   class Entry
-    STORED   = 0
-    DEFLATED = 8
+    STORED   = ::Zip::COMPRESSION_METHOD_STORE
+    DEFLATED = ::Zip::COMPRESSION_METHOD_DEFLATE
+
     # Language encoding flag (EFS) bit
     EFS = 0b100000000000