Update INSTALL.ubuntu1604.txt to install pip3
Update INSTALL.ubuntu1604.txt to install pip3

System complains about missing pip3 when attempting to install support for STIX 2.0 (cf. line 88):

ubuntu@misp:/var/www/MISP/app/files/scripts/mixbox$ pip3 install stix2
The program 'pip3' is currently not installed. You can install it by typing:
sudo apt install python3-pip

Therefore suggest to include installation of python3-pip in previous instance of apt-get usage (line 69)
FIRSTdotorg/MISP,MISP/MISP,MISP/MISP,SteveClement/MISP,SteveClement/MISP,devnull-/MISP,Rafiot/MISP,FIRSTdotorg/MISP,FloatingGhost/MISP,SteveClement/MISP,Rafiot/MISP,Rafiot/MISP,MISP/MISP,FloatingGhost/MISP,MISP/MISP,SteveClement/MISP,Rafiot/MISP,FloatingGhost/MISP,devnull-/MISP,devnull-/MISP,devnull-/MISP,TheDr1ver/MISP,SteveClement/MISP,FloatingGhost/MISP,TheDr1ver/MISP,TheDr1ver/MISP,TheDr1ver/MISP,MISP/MISP,FIRSTdotorg/MISP,FIRSTdotorg/MISP