travis: use matching C++ for GCC version

travis: use matching C++ for GCC version

When we use GCC 5.x, we need to install the C++ compiler and the C
compiler together because QEMU tests for feature flags against the C
compiler and assumes the C++ compiler has them. We also have to
ensure that GCC C++ is used. Have to do the modification of the CXX variable
in two steps to ensure we support older versions of bash in use by the
test machines. While we're at it simply how we select our compiler.

Signed-off-by: Doug Goldstein <cardoe@cardoe.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>