Avoid requiring krb5 in migration

This simplifies source installations.

The Kerberos gem is not required to be installed, so don't fail
if we can't load this in the migration.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/7980.
Signed-off-by: Rémy Coutable <remy@rymai.me>

ee/db/migrate/20151113115819_canonicalize_kerberos_identities.rb

@@ -12,7 +12,7 @@ class CanonicalizeKerberosIdentities < ActiveRecord::Migration
       default_realm = krb5.get_default_realm
       krb5.close # release memory allocated by the krb5 library
       default_realm || ''
-    rescue StandardError
+    rescue StandardError, LoadError
       '' # could not find the system's default realm, maybe there's no Kerberos at all
     end
   end