update ver requirements in docs

Author: darioizzo

doc/install.rst

@@ -129,9 +129,9 @@ In order to install pygmo from source, you will need:
   Clang or MSVC should do),
 * a `Python <https://www.python.org/>`__ installation,
 * `pybind11 <https://github.com/pybind/pybind11>`__ (version >= 2.10),
-* the `pagmo C++ library <https://esa.github.io/pagmo2/>`__,
+* the `pagmo C++ library <https://esa.github.io/pagmo2/>`__, (version >=2.19)
 * the `Boost libraries <https://www.boost.org/>`__,
-* `CMake <https://cmake.org/>`__, version 3.17 or later.
+* `CMake <https://cmake.org/>`__, version 3.18 or later.
 
 After making sure the dependencies are installed on your system, you can
 download the pygmo source code from the