Use Ubuntu Trusty on Travis for a new CMake

(cherry picked from commit 0284a2893a)

.travis.yml

@@ -1,4 +1,5 @@
 language: cpp
+dist: trusty
 sudo: required # the doc target uses sudo to install dependencies
 
 os:
@@ -22,12 +23,5 @@ matrix:
     - os: osx
       env: BUILD=Doc
 
-addons:
-  apt:
-    sources:
-      - kubuntu-backports # cmake 2.8.12
-    packages:
-      - cmake
-
 script:
   - support/travis-build.py