Update musl in CI

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

@@ -1,5 +1,7 @@
 FROM ubuntu:17.10
 
+ENV MUSL_VERSION 1.1.16
+
 RUN dpkg --add-architecture i386
 RUN apt-get update
 RUN apt-get install -y --no-install-recommends \
@@ -12,13 +14,13 @@ RUN apt-get install -y --no-install-recommends \
 #   since otherwise the script will fail to find a compiler.
 # * We manually unset CROSS_COMPILE when running make; otherwise the makefile
 #   will call the non-existent binary 'i686-ar'.
-RUN curl https://www.musl-libc.org/releases/musl-1.1.15.tar.gz | \
+RUN curl https://www.musl-libc.org/releases/musl-$MUSL_VERSION.tar.gz | \
     tar xzf - && \
-    cd musl-1.1.15 && \
+    cd musl-$MUSL_VERSION && \
     CC=gcc CFLAGS=-m32 ./configure --prefix=/musl-i686 --disable-shared --target=i686 && \
     make CROSS_COMPILE= install -j4 && \
     cd .. && \
-    rm -rf musl-1.1.15 && \
+    rm -rf musl-$MUSL_VERSION && \
 # Install linux kernel headers sanitized for use with musl
     curl -L  https://github.com/sabotage-linux/kernel-headers/archive/v3.12.6-5.tar.gz | \
     tar xzf - && \

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

@@ -1,15 +1,17 @@
 FROM ubuntu:17.10
 
+ENV MUSL_VERSION 1.1.16
+
 RUN apt-get update
 RUN apt-get install -y --no-install-recommends \
   gcc make libc6-dev git curl ca-certificates
-RUN curl https://www.musl-libc.org/releases/musl-1.1.15.tar.gz | \
+RUN curl https://www.musl-libc.org/releases/musl-$MUSL_VERSION.tar.gz | \
     tar xzf - && \
-    cd musl-1.1.15 && \
+    cd musl-$MUSL_VERSION && \
     ./configure --prefix=/musl-x86_64 && \
     make install -j4 && \
     cd .. && \
-    rm -rf musl-1.1.15 && \
+    rm -rf $MUSL_VERSION && \
 # Install linux kernel headers sanitized for use with musl
     curl -L  https://github.com/sabotage-linux/kernel-headers/archive/v3.12.6-5.tar.gz | \
     tar xzf - && \