Port the Dockerfile used by travis to use python3

Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>

Author: pypingou

.travis-Dockerfile

@@ -12,16 +12,16 @@ RUN yum -y update && \
         gcc-c++ \
         krb5-devel \
         libffi-devel \
-        libyaml-devel \
+#        libyaml-devel \
         openssl-devel \
-        python27 \
-        python-devel \
-        python-pip \
+        python3 \
+        python3-devel \
+        python3-pip \
         redhat-rpm-config \
         swig \
         zeromq-devel \
         rpm-devel \
-        python2-koji \
+        python3-koji \
         libmodulemd && \
     yum clean all