fix SecurityError on connect with $SAFE=1

If core.rb was required without raising a SecurityError, the directory
it was loaded from is trusted enough to untaint.

lib/sequel/core.rb

@@ -188,7 +188,7 @@ def self.quote_identifiers=(value)
   # This is used to ensure that the files loaded are from the same version of
   # Sequel as this file.
   def self.require(files, subdir=nil)
-    Array(files).each{|f| super("#{File.dirname(__FILE__)}/#{"#{subdir}/" if subdir}#{f}")}
+    Array(files).each{|f| super("#{File.dirname(__FILE__).untaint}/#{"#{subdir}/" if subdir}#{f}")}
   end
 
   # Set whether to set the single threaded mode for all databases by default. By default,