Fix issues with gcc 5.0, or deprecate.

[AdamBrouwersHarries]

At the syclparallelstl fails to build on travis due to compile errors with gcc 5.0. These should either be fixed, or gcc 5.0 should be deprecated in order to have a clean build on travis - at present it is hard to tell at a glance whether there are any issues with the SyclParallelSTL or not.

[keryell]

I propose to deprecate.
After all, the SYCL ParallelSTL is about an implementation of the C++17 Parallel STL, so skipping a compiler from 2015 does not shock me.

[keryell]

@agozillon could you look at why it does not compile with triSYCL?

It looks like the the CI is using a Jurassic environment... Start with updating the CI with triSYCL with Ubuntu 18.10, g++ 8, clang++ 7. Pick some configuration pieces from triSYCL CI configuration.

[Ruyk]

I've bumped the clang version to the same one you have in master triSYCL:

#89