Omit --homedir for symmetric encryption, to work around gpg2 bug

GnuPG 2.0.x contains a bug wherein providing a --homedir option to
symmetric encryption (which is not necessary, as there are no keys
involved) encrypts successfully, but errors because it cannot connect to
the (unused) GPG agent:

    https://bugs.g10code.com/gnupg/issue1427
    https://bugzilla.redhat.com/show_bug.cgi?format=multiple&id=841588

Work around this issue by clearing the homedir, if set, before symmetric
encryption, and setting back to its previous value afterward.
Checking in changes prior to tagging of version 0.51.

Changelog diff is:

diff --git a/Changes b/Changes
index 630ff3b..31e4d36 100644
--- a/Changes
+++ b/Changes
@@ -1,5 +1,10 @@
 Revision history for GnuPG-Interface

+0.51 - 2014-12-17
+ - Update README file
+ - Work around gpg2 bug by omitting --homedir during symmetric
+   encryption
+
 0.50 - 2014-03-14
  - Version 0.49 implicitly required Moose; switch to a technique that
    does not