Fix testing dockerfiles for 14.04 and 15.10: the python3-sh package didn't exist at that time, install it via pip instead.
diff --git a/extras/dockerfiles/ubuntu-14.04_install.sh b/extras/dockerfiles/ubuntu-14.04_install.sh
index d9628ae..30a4365 100644
--- a/extras/dockerfiles/ubuntu-14.04_install.sh
+++ b/extras/dockerfiles/ubuntu-14.04_install.sh
@@ -1,3 +1,6 @@
 #!/bin/bash
 
 set -e
+
+# python3-sh doesn't exist in Ubuntu 14.04, we must install it via pip instead.
+pip3 install --user sh