Kaldi now requires python2 *and* python3

Add python3 to the list of dependencies, as it is now required by
Kaldi `check_dependencies.sh` script.

This change was introduced in Kaldi at:
kaldi-asr/kaldi@8ad898c

Author: tomjorquera

Dockerfile

@@ -18,6 +18,7 @@ RUN apt-get update && apt-get install -y  \
     libtool-bin \
     make \
     python2.7 \
+    python3 \
     python-pip \
     python-yaml \
     python-simplejson \