Increase required CMake version to 3.8

[brycelelbach]

pushmi uses the CMake cxx_std_XX compile features, which were introduced in CMake 3.7. However, pushmi currently only requires CMake version 3.7. So, if you happen to be using CMake version 3.7, running CMake fails with a cryptic error:

$ cmake --version
cmake version 3.7.2

CMake suite maintained and supported by Kitware (kitware.com/cmake).

$ /home/wash/install/nvidia/cuda-9.2.88/bin/nvcc --version
nvcc: NVIDIA (R) Cuda compiler driver
Copyright (c) 2005-2018 NVIDIA Corporation
Built on Wed_Apr_11_23:16:29_CDT_2018
Cuda compilation tools, release 9.2, V9.2.88

$ g++ --version
g++ (Debian 6.4.0-12) 6.4.0 20180123
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ git rev-parse HEAD
cedca49cdc60413c35f64710eb5ea3987111b380

$ ls
buildSingleHeader.cmake  CMakeLists.txt  CODE_OF_CONDUCT.md  CONTRIBUTING.md  examples  external  include  ISSUES  LICENSE  Readme.md  TARGETS  test

$ mkdir build

$ cd build

$ cmake -DCMAKE_CXX_COMPILER=/home/wash/install/nvidia/cuda-9.2.88/bin/nvcc ..
-- The CXX compiler identification is GNU 6.4.0
-- Check for working CXX compiler: /home/wash/install/nvidia/cuda-9.2.88/bin/nvcc
-- Check for working CXX compiler: /home/wash/install/nvidia/cuda-9.2.88/bin/nvcc -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - failed
-- Looking for C++ include pthread.h
-- Looking for C++ include pthread.h - found
-- Looking for pthread_create
-- Looking for pthread_create - found
-- Found Threads: TRUE  
-- Configuring done
CMake Error in examples/twoway_execute/CMakeLists.txt:
  Specified unknown feature "cxx_std_14" for target "twoway_execute_2".


CMake Error in examples/then_execute/CMakeLists.txt:
  Specified unknown feature "cxx_std_14" for target "then_execute_2".


CMake Error in examples/composition/CMakeLists.txt:
  Specified unknown feature "cxx_std_14" for target "composition_3".


CMake Error in examples/composition/CMakeLists.txt:
  Specified unknown feature "cxx_std_14" for target "composition_2".


CMake Error in examples/composition/CMakeLists.txt:
  Specified unknown feature "cxx_std_14" for target "composition_4".


CMake Error in examples/for_each/CMakeLists.txt:
  Specified unknown feature "cxx_std_14" for target "for_each_3".


CMake Error in examples/for_each/CMakeLists.txt:
  Specified unknown feature "cxx_std_14" for target "for_each_2".


CMake Error in examples/reduce/CMakeLists.txt:
  Specified unknown feature "cxx_std_14" for target "reduce_3".


CMake Error in examples/reduce/CMakeLists.txt:
  Specified unknown feature "cxx_std_14" for target "reduce_2".


CMake Error in test/CMakeLists.txt:
  Specified unknown feature "cxx_std_14" for target "PushmiTest".


-- Generating done
-- Build files have been written to: /home/wash/development/iso-c++/pushmi/build

This PR increases the minimum CMake version to 3.8.

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