travis-ci: install cmake from xenial (CTEST_PARALLEL_LEVEL support)

cmake supports it since 3.0 I guess.

And it must be trusty (not precise), otherwise it fails:
  dpkg: error processing
  /var/cache/apt/archives/ifupdown_0.8.10ubuntu1_amd64.deb (--unpack):
   trying to overwrite '/etc/init.d/networking', which is also in package netbase 4.47ubuntu1

Link: https://travis-ci.org/azat/libevent/jobs/193744023

.travis.yml

@@ -2,6 +2,7 @@ os:
   - linux
   - osx
 sudo: false
+dist: trusty
 
 env:
   matrix:
@@ -47,14 +48,17 @@ before_install:
 
 addons:
   apt:
+    sources:
+    - xenial
+    - sourceline: 'deb http://archive.ubuntu.com/ubuntu xenial main'
     packages:
-      - zlib1g-dev
-      - libssl-dev
-      - build-essential
-      - automake
-      - autoconf
-      - cmake
-      - lcov
+    - zlib1g-dev
+    - libssl-dev
+    - build-essential
+    - automake
+    - autoconf
+    - cmake
+    - lcov
 
 script:
   - if [ "$EVENT_BUILD_METHOD" = "autotools" ]; then