Fix travis

The LLVM APT repo has been turned off due to excessive load.
To avoid pulling directly from the LLVM APT repo, use a version of
Ubuntu that contains the nesseary compilers. Xenial is the next LTS
version of Ubuntu and
[contains](http://packages.ubuntu.com/Xenial/devel/) clang 3.6

This patch follows the suggestion
[here](travis-ci/travis-ci#6120 (comment))

.travis.yml

@@ -1,6 +1,7 @@
 sudo: false
 
 language: cpp
+dist: xenial
 
 env:
   global:
@@ -13,7 +14,6 @@ addons:
   apt:
     sources: &common_sources
       - ubuntu-toolchain-r-test
-      - llvm-toolchain-precise-3.6
       - kalakris-cmake
     packages: &common_packages
       - autoconf