update docs: c++17 now required?

[Charles Ferenbaugh]

Does flecsi now require c++17 to compile? I tried compiling the current master against gcc 6.4.0 and got a build error for an inline variable, which is new to c++17. I had to move up to gcc 7.3.0 to build.

If c++17 is required now, the README needs to be updated accordingly.

Edit: I thought I recalled seeing a commit from @ktsai7 to upgrade to c++17, but I can't find that commit now.

[Karen C. Tsai]

Yes, Ben updated FleCSI to C++17 few weeks (months?) ago. Then libristra got up to C++17 shortly after by someone else, so I did a commit to update the cinch module for compilation problems that we started having.

I will go through and update FleCSI README accordingly.
@tuxfan Is there anywhere else that might need to be double checked other than READMEs?

[Charles Ferenbaugh]

Thanks. Note that in addition to changing c++14 -> c++17, we also need gcc 6 -> gcc 7.