Derive SBT Docker image from debian:jessie

Ubuntu docker image for Gerrit build is obsolete: use
the new debian:jessie as base image from now on.

Change-Id: Ic696c707fc9a2419e9fd7df1e2a860c9ca4da5ff
diff --git a/jenkins-docker/slave-sbt/Dockerfile b/jenkins-docker/slave-sbt/Dockerfile
index a3e836a..90126df 100644
--- a/jenkins-docker/slave-sbt/Dockerfile
+++ b/jenkins-docker/slave-sbt/Dockerfile
@@ -1,8 +1,8 @@
-FROM gerritforge/gerrit-ci-slave-bazel
+FROM gerritforge/gerrit-ci-slave-bazel:debian-jessie
 
 # SBT Setup
 RUN apt-get update && \
-    apt-get -y install bc apt-transport-https && \
+    apt-get -y install apt-transport-https && \
     echo "deb https://dl.bintray.com/sbt/debian /" | tee -a /etc/apt/sources.list.d/sbt.list && \
     apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 642AC823 && \
     apt-get update && \