Upgrade to use latest clang

[springmeyer]

• On linux we use clang-3.8 via mason
• On OS X we use latest clang from xcode

This unlocks moving to c++14 because both compilers provide headers recent enough for c++14 features. For linux this is enabled by using g++-5 headers (clang++ on linux depends on g++ being installed for its headers). On OS X this is enabled by using the latest Xcode.

For binaries built this way on linux users will need to upgrade libstdc++ on both Ubuntu Precise and Trusty.

For boxes with access to sudo and PPAs:

add-apt-repository -y ppa:ubuntu-toolchain-r/test
apt-get update -y
apt-get install -y libstdc++6

For travis machines that only need to run C++ binaries (not compile them):

addons:
  apt:
    sources:
     - ubuntu-toolchain-r-test
    packages:
     - libstdc++6

[TheMarex]

Looks good to me. :+1: