change travis CI distro

.travis.yml

@@ -1,9 +1,11 @@
+dist: xenial
 language: cpp
 compiler:
 - gcc
 before_install:
 - sudo add-apt-repository --yes ppa:ubuntu-sdk-team/ppa
 - sudo apt-get update -qq
+- sudo apt-get install libgnutls28-dev
 - sudo apt-get install build-essential pkg-config libcurl3-gnutls-dev libc6-dev libevent-dev
   m4 g++-multilib autoconf libtool ncurses-dev unzip git python zlib1g-dev wget bsdmainutils
   automake libssl-dev libprotobuf-dev protobuf-compiler libdb++-dev ntp ntpdate