Update FreeBSD docker CI to use FreeBSD 11.1 image

ci/docker/x86_64-unknown-freebsd/Dockerfile

-FROM alexcrichton/port-prebuilt-freebsd:2017-09-16
+FROM wezm/port-prebuilt-freebsd11@sha256:43553e2265ec702ec72a63a765df333f50b1858b896e69385749e96d8624e9b0
 
 RUN apt-get update
 RUN apt-get install -y --no-install-recommends \
-  qemu genext2fs
+  qemu genext2fs xz-utils
 RUN apt-get install -y curl ca-certificates gcc
 
 ENTRYPOINT ["sh"]
 
 ENV PATH=$PATH:/rust/bin \
-    QEMU=2016-11-06/freebsd.qcow2.gz \
+    QEMU=2018-03-15/FreeBSD-11.1-RELEASE-amd64.qcow2.xz \
     CAN_CROSS=1 \
-    CARGO_TARGET_X86_64_UNKNOWN_FREEBSD_LINKER=x86_64-unknown-freebsd10-gcc
+    CARGO_TARGET_X86_64_UNKNOWN_FREEBSD_LINKER=x86_64-unknown-freebsd11-gcc

ci/run.sh

@@ -24,6 +24,13 @@ if [ "$QEMU" != "" ]; then
       curl https://s3-us-west-1.amazonaws.com/rust-lang-ci2/libc/$QEMU | \
         gunzip -d > $tmpdir/$qemufile
     fi
+  elif [ -z "${QEMU#*.xz}" ]; then
+    # image is .xz : download and uncompress it
+    qemufile=$(echo ${QEMU%.xz} | sed 's/\//__/g')
+    if [ ! -f $tmpdir/$qemufile ]; then
+      curl https://s3-us-west-1.amazonaws.com/rust-lang-ci2/libc/$QEMU | \
+        unxz > $tmpdir/$qemufile
+    fi
   else
     # plain qcow2 image: just download it
     qemufile=$(echo ${QEMU} | sed 's/\//__/g')