Set C++17 standard by default, upgrade to Clang 7 and GCC 8

We're now using C++17 by default in Athena. When compiling ACTS v0.07.03 with Clang 7 and C++17 we see an error:

http://atlas-computing.web.cern.ch/atlas-computing/links/distDirectory/gitwww/GITWebArea/nightlies/master/2019-02-11T1053/Athena/x86_64-slc6-clang70-opt/AthenaExternals/External.Acts.log.html

I couldn't reproduce this with a newer tag or master, so it seems to be fixed. But it might be good on the ACTS side to update compilers and standards in the meantime.

Cheers,

Stewart