Tests: avoid using encode_base64url().

It only recently appeared in MIME::Base64 3.11 and only shipped
with Perl 5.13.8+.
diff --git a/secure_link.t b/secure_link.t
index 50177e5..e3a9933 100644
--- a/secure_link.t
+++ b/secure_link.t
@@ -12,7 +12,7 @@
 use Test::More;
 
 use Digest::MD5 qw/ md5 md5_hex /;
-use MIME::Base64 qw/ encode_base64url /;
+use MIME::Base64 qw/ encode_base64 /;
 
 BEGIN { use FindBin; chdir($FindBin::Bin); }
 
@@ -136,3 +136,12 @@
 like(http_get('/p/test.html'), qr/^HTTP.*403/, 'request old style no hash');
 
 ###############################################################################
+
+sub encode_base64url {
+    my $e = encode_base64(shift, "");
+    $e =~ s/=+\z//;
+    $e =~ tr[+/][-_];
+    return $e;
+}
+
+###############################################################################