Move some containers to ubuntu 18.04

Apparently 17.04 is broken now?

ci/docker/i686-unknown-linux-gnu/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:17.04
+FROM ubuntu:18.04
 RUN apt-get update
 RUN apt-get install -y --no-install-recommends \
   gcc-multilib libc6-dev ca-certificates

ci/docker/x86_64-unknown-linux-gnu/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:17.04
+FROM ubuntu:18.04
 RUN apt-get update
 RUN apt-get install -y --no-install-recommends \
   gcc libc6-dev ca-certificates

ci/docker/x86_64-unknown-linux-gnux32/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:17.04
+FROM ubuntu:18.04
 RUN apt-get update
 RUN apt-get install -y --no-install-recommends \
   gcc-multilib libc6-dev ca-certificates

libc-test/build.rs

@@ -555,6 +555,11 @@ fn main() {
             // These are defined for Solaris 11, but the crate is tested on illumos, where they are currently not defined
             "EADI" | "PORT_SOURCE_POSTWAIT" | "PORT_SOURCE_SIGNAL" | "PTHREAD_STACK_MIN" => true,
 
+            // These change all the time from release to release of linux
+            // distros, let's just not bother trying to verify them. They
+            // shouldn't be used in code anyway...
+            "AF_MAX" | "PF_MAX" => true,
+
             _ => false,
         }
     });