Are you still having trouble with this? I had to do the following before I could run Let’s Encrypt in AWS:
sudo yum install git
sudo yum install libffi-devel
sudo yum install puppet
cd /var/app/current
git clone https://github.com/letsencrypt/letsencrypt
cd letsencrypt
./letsencrypt-auto --server https://acme-v01.api.letsencrypt.org/directory --help
./bootstrap/venv.sh
My server actually had python2.7 installed already but Let’s Encrypt wouldn’t recognise it (I had to add the debug flag a lot). Have you had similar issues since installing python?