travis: Remove clang-3.8 build

travis: Remove clang-3.8 build

The package appears to have been renamed in Ubuntu.  The only reason this test
is currently passing is because the hypervisor build falls back to clang, at
version 3.5

Add an explicit test in the build script that out desired compiler is
available.  Note that travis already performs this step, but in a way which
isn't fatal to the build.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by Doug Goldstein <cardoe@cardoe.com>