update raspberrypi3 install procedure (#148)

install stable version of libreadline6 based on comment from MaxTG

contrib/raspberrypi3/install_electrumx.sh

@@ -11,7 +11,8 @@ sudo apt-get autoremove
 # install electrumx dependencies
 sudo apt-get install python3-pip
 sudo apt-get install build-essential libc6-dev
-sudo apt-get install libncurses5-dev libncursesw5-dev libreadline6-dev
+sudo apt-get install libncurses5-dev libncursesw5-dev
+sudo apt install libreadline6-dev/stable libreadline6/stable
 sudo apt-get install libleveldb-dev
 sudo apt-get install git
 sudo pip3 install plyvel