Build with clang and g++-4.8 on travis

In order to build with clang, we have to compromise a bit and build with
embedded boost. Due to c++11 changes, boost libraries built with g++ can
cause issues when building with clang, if by issues I mean segfaults.
Therefore, we build with embedded boost when using clang.

Clang c++11 support does require updated libstdc (on Travis) so we
install g++-4.8 regardless of our compiler choice.

Since it was easy, this patch also updates our g++ version to 4.8.
Merge pull request #388 from gadomski/clang

Travis, build clang
Use ninja when building with clang

Our clang side of the travis matrix is going to be our 'funky stuff
zone'. We also install cmake 2.8.12.2 for the Ninja generator.
Merge pull request #389 from gadomski/travis-clang-ninja

Use ninja when building with clang
Disable Ninja builds for clang

The cmake build was taking too long, so Travis was killing it. If we
find a packaged cmake 2.8.12 we could re-enable Ninja.
Re-enable Ninja, but set the thread count
Merge branch 'master' of https://github.com/PDAL/PDAL into point-context