small/perf build fails

CMakeLists.txt for the project sets C++version to 11. Benchmark now switched to C++ 14 and used std::make_unique<>. Build now fails due to this:

<...>/include/benchmark/benchmark.h: In function ‘benchmark::internal::Benchmark* benchmark::RegisterBenchmark(const string&, void (*)(benchmark::State&))’:
<...>/include/benchmark/benchmark.h:1371:12: error: ‘make_unique’ is not a member of ‘std’
