Fix build instructions for ubuntu.

Author: kpeeters

README.rst

@@ -60,7 +60,7 @@ On Debian/Ubuntu you can install all that is needed with::
     sudo apt install git cmake python3-dev g++ libpcre3 libpcre3-dev libgmp3-dev \
           libgtkmm-3.0-dev libboost-all-dev libgmp-dev libsqlite3-dev uuid-dev  \
           texlive texlive-latex-extra dvipng \
-          python3-matplotlib python3-mpmath python3-sympy
+          python3-matplotlib python3-mpmath python3-sympy python3-gmpy2
 
 (on Ubuntu 14.04 you need to replace `cmake` with `cmake3` and also
 install g++-4.9; get in touch if you don't know how to do this). On