Temp. fix for llvm apt being down

[jszuppe]

Apt for llvm and clang has been down for quite some time (see travis-ci/travis-ci#6120), so here is the fix for POCL builds which require the latest clang and llvm.

[coveralls]

Coverage remained the same at 80.861% when pulling a721db2 on haahh:pr_pocl_fix_llvm_apt into 04e7dde on boostorg:develop.

[kylelutz]

Thanks!