bump minimum cmake version required

Currently the CMakeLists.txt specifies:

cmake_minimum_required(VERSION 2.8)

However on this line:

https://github.com/couchbase/forestdb/blob/master/CMakeLists.txt#L206

We make use of the "OBJECT" library type, which according to this page:

https://cmake.org/Wiki/CMake/Tutorials/Object_Library

was not introduced until version 2.8.8.

On our travis-ci infrastructure, we had version: cmake version 2.8.7 and we would get errors of the form:

CMake Error at CMakeLists.txt:206 (add_library):

  Cannot find source file:

    OBJECT

  Tried extensions .c .C .c++ .cc .cpp .cxx .m .M .mm .h .hh .h++ .hm .hpp

  .hxx .in .txx

Once we upgraded this problem went away. If we can bump the minimum version required, it will help other users avoid this confusion in the future.