The default build environment has been updated from Trusty to Xenial.
To keep using Trusty you can set: dist: trusty
Or use Xenial and remove the llvm-toolchain-trusty-5.0 etc. since clang-5.0 (and 4.0 and 6.0) are available in the default Apt packages. clang-5.0 @ packages.ubuntu.com.
Note that Trusty is EOL.