Install curl in Ubuntu requirements

Ubuntu does not ship with curl by default any more. If we are to use
curl to install SBT's required information, we must ensure it is
available for use on Ubuntu.

scripts/ubuntu-req.sh

@@ -2,6 +2,7 @@
 
 set -ex
 
+sudo apt-get install -y curl
 sudo apt-get install -y build-essential bison flex
 sudo apt-get install -y libgmp-dev libmpfr-dev libmpc-dev zlib1g-dev vim git default-jdk default-jre
 # install sbt: https://www.scala-sbt.org/release/docs/Installing-sbt-on-Linux.html#Ubuntu+and+other+Debian-based+distributions