travis: Compile with C++11 support with cmake on clang.

We can not add it to gcc yet, as gcc 4.6, as is used by the travis
server, does not support it yet.
Remove nothrow specifier hack for apple.

The __GLIBCXX_THROW defines itself as nothing for C++11, as the throw
specifiers are deprecated in C++11. This will not be changed to the same
type specifiers as is on C++03, and should be a problem for every C++11
compatible compiler.

A previous patch fixes this generally for C++11 compilers. Not only on
apple with glibc.