BUILD: travis-ci: limit build to branches "master" and "next"

Occasionally some short-lived branches are pushed to help developers
rebase their work, these ones do not need to be built. This patch
explicitly lists "master" and "next" as the two only branches of
interest. It also adds a comment with the URL for the build status.

.travis.yml

@@ -1,8 +1,15 @@
+# build status appears on https://travis-ci.com/haproxy/haproxy
+
 sudo: required
 dist: xenial
 
 language: c
 
+branches:
+  only:
+    - master
+    - next
+
 env:
   global:
     - FLAGS="USE_ZLIB=1 USE_PCRE=1 USE_PCRE_JIT=1 USE_LUA=1 USE_OPENSSL=1 USE_SYSTEMD=1 USE_WURFL=1 WURFL_INC=contrib/wurfl WURFL_LIB=contrib/wurfl USE_DEVICEATLAS=1 DEVICEATLAS_SRC=contrib/deviceatlas USE_51DEGREES=1"