Added prerequisites.sh step to install requests package

toolset/setup/linux/prerequisites.sh

@@ -60,6 +60,7 @@ sudo pip install colorama==0.3.1
 # Version 2.3 has a nice Counter() and other features
 # but it requires —-allow-external and -—allow-unverified
 sudo pip install progressbar==2.2
+sudo pip install requests
 
 # Install gcc-4.8 and gcc-4.9
 sudo add-apt-repository ppa:ubuntu-toolchain-r/test -y