upgrade cmake version to 3.9

[jouho]

Release Summary:

Updated CMake version from 3.0 to 3.9.

Resolved issues:

resolves #4899

Description of changes:

Updated the cmake_minimum_required version from 3.0 to 3.9.

Call-outs:

Testing:

Before this change, you'd see following warning when you build s2n-tls:

~/s2n-tls$ cmake . -Bbuild     -DCMAKE_BUILD_TYPE=Release     -DCMAKE_INSTALL_PREFIX=./s2n-tls-install
CMake Deprecation Warning at CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.5 will be removed from a future version of
  CMake.

  Update the VERSION argument <min> value or use a ...<max> suffix to tell
  CMake that the project does not need compatibility with older versions.


-- Detected CMAKE_SYSTEM_PROCESSOR as x86_64
-- Detected 64-Bit system
-- LibCrypto Include Dir: /usr/include
...

After this change, the warning message is gone:

~/s2n-tls$ cmake . -Bbuild     -DCMAKE_BUILD_TYPE=Release     -DCMAKE_INSTALL_PREFIX=./s2n-tls-install
-- Detected CMAKE_SYSTEM_PROCESSOR as x86_64
-- Detected 64-Bit system
-- LibCrypto Include Dir: /usr/include
...

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[maddeleine]

Should be fine as long as all tests pass.

[boquan-fang]

All tests (except the team labeler) pass. Hence, approve this PR.