All CI integrations on Travis

[MatteoRagni]

This commit removes appveyor and launches all CI on Travis with the following workers:

• Linux gcc-4.9
• Linux gcc-5.3
• Linux gcc-7.3
• Linux llvm
• Osx xcode-9
• Osx xcode-10
• Windows mingw 4.9.3 (Currently used by Matlab)
• Windows mingw 5.3
• Windows VS2015 via cmake