Use `Priority: optional` and reorder some fields

`Priority: extra` is deprecated
https://www.debian.org/doc/debian-policy/ch-archive.html#s-priorities

Change-Id: I2cfa0432240452a8fdd5e0b4a2fb8b4c908db612
diff --git a/repo-ng/debian/control b/repo-ng/debian/control
index 4faf4b1..f680725 100644
--- a/repo-ng/debian/control
+++ b/repo-ng/debian/control
@@ -1,19 +1,19 @@
 Source: repo-ng
-Priority: extra
+Section: net
+Priority: optional
 Maintainer: Alexander Afanasyev <alexander.afanasyev@ucla.edu>
 Build-Depends: debhelper (>= 9.20160709),
  python3 (>= 3.6.0),
  pkg-config (>= 0.26),
  libndn-cxx-dev (>= 0.8.0)
 Standards-Version: 3.9.2
-Section: net
-Homepage: http://github.com/named-data/repo-ng
-Vcs-Git: git://github.com/named-data/repo-ng.git
-Vcs-Browser: http://github.com/named-data/repo-ng
+Homepage: https://github.com/named-data/repo-ng
+Vcs-Browser: https://github.com/named-data/repo-ng
+Vcs-Git: https://github.com/named-data/repo-ng.git
 
 Package: repo-ng
 Architecture: i386 amd64 arm64 armel armhf ppc64el
 Depends: ${shlibs:Depends}, ${misc:Depends}, libndn-cxx, ndnsec
-Description: Next generation of NDN repository
+Description: next generation of NDN repository
  repo-ng is an implementation of a Named Data Networking data repository with the protocol
  defined on NDN wiki: http://redmine.named-data.net/projects/repo-ng/wiki