* travis.yml clean up
- Remove trailling spaces
- Move notification setting from the bottom to the top
- Set Linux as default OS
- Simplify cache directory
- Add banners around different configurations
* Update CMake installation routine
- Make it depend on variable (easier to update)
- Simplify code a bit
* Re-organize build matrix and LLVM install
- make logic depend on a single COMPILER variable
- Update LLVM installation process
* Update compilers to latest stable version
* CI updates:
Travis: use cmake 3.7
Appveyor: Add .appveyor.yml config, test VS2013/VS2015/VS2017 x86/x64 Debug/Release.
* Drop VS2013 support from AppVeyor.