CONTRIB: move 51Degrees to addons/51degrees

Now it's much cleaner, both 51d.c and the dummy library live together and
are easier to spot and maintain. The build howto probably ought to be moved
there as well. Makefile, docs and MAINTAINERS were updated, as well as
the github CI's build matrix, travis CI's, and coverity checks.
diff --git a/MAINTAINERS b/MAINTAINERS
index 7f4881d..144cc42 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -20,7 +20,7 @@
 
 51Degrees device identification
 Maintainer: Ben Shillito <ben@51degrees.com>
-Files: src/51d.c, contrib/51d, doc/51Degrees-device-detection.txt
+Files: addons/51degrees, doc/51Degrees-device-detection.txt
 
 Cache
 Maintainer: William Lallemand <wlallemand@haproxy.com>