Fix build on CentOS

[sandra-thieme]

Issue

Fixes build on CentOS systems which doesn't have gcc ready for C++14 yet.

Fixes issue #2804

[TheMarex]

Doesn't c++1y mean c++11 on older compilers? We switched to C++14 a while ago and just didn't get around to clean up the legacy code that backported some 14 functionality (e.g. http://en.cppreference.com/w/cpp/memory/unique_ptr/make_unique)

[MoKob]

From what I've read c++1y is the respective draft of c++14 just like c++11 was referred to by c++0x. Using a draft only does not seem a valid step in our progression to me.

[sandra-thieme]

This is a change proposed by @daniel-j-h yesterday in irc:

11:56:40 <   danieljh> rkcpi: if it works fine with "c++1y" instead can you open a pull request with that change and see how travis likes it?

Is there another valid approach to still be able to build with gcc on CentOS?

[TheMarex]

I'm fine with merging this, I confused c++0x with c++1y. Just note that builds for older compiler could break.

[TheMarex]

Merging here. Note that on newer compiler c++1y is just an alias for c++14 so it doesn't change anything for our current setup.