ci: Use Ubuntu 18.04 rather than Ubuntu 16.04

We will soon require Ubuntu 18.04 rather than Ubuntu 16.04 for building
op-build due to a GCC6.2 requirement from glibc.

Signed-off-by: Stewart Smith <stewart@linux.ibm.com>
diff --git a/ci/build.sh b/ci/build.sh
index 9359ca6..8a275f3 100755
--- a/ci/build.sh
+++ b/ci/build.sh
@@ -1,6 +1,6 @@
 #!/bin/bash
 
-CONTAINERS="ubuntu1604 fedora27"
+CONTAINERS="ubuntu1804 fedora27"
 
 
 SDK_ONLY=0
@@ -104,7 +104,7 @@
 	  if [[ "$distro" == fedora27 ]]; then
 	    PROXY="RUN echo \"proxy=${http_proxy}\" >> /etc/dnf/dnf.conf"
 	  fi
-	  if [[ "$distro" == ubuntu1604 ]]; then
+	  if [[ "$distro" == ubuntu1804 ]]; then
 	    PROXY="RUN echo \"Acquire::http::Proxy \\"\"${http_proxy}/\\"\";\" > /etc/apt/apt.conf.d/000apt-cacher-ng-proxy"
 	  fi
         fi