Update the mips docker image

Use the standard mips compilers from Ubuntu now

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

-FROM ubuntu:15.10
+FROM ubuntu:16.04
 
 RUN apt-get update
 RUN apt-get install -y --no-install-recommends \
-        software-properties-common gcc libc6-dev qemu-user
-RUN add-apt-repository ppa:angelsl/mips-cross
-RUN apt-get update
-RUN apt-get install -y --no-install-recommends \
-        gcc-5-mips-linux-gnu libc6-dev-mips-cross
+        gcc libc6-dev qemu-user ca-certificates \
+        gcc-mips-linux-gnu libc6-dev-mips-cross \
+        qemu-system-mips
 
-ENV CARGO_TARGET_MIPS_UNKNOWN_LINUX_GNU_LINKER=mips-linux-gnu-gcc-5 \
-    CC_mips_unknown_linux_gnu=mips-linux-gnu-gcc-5 \
+ENV CARGO_TARGET_MIPS_UNKNOWN_LINUX_GNU_LINKER=mips-linux-gnu-gcc \
     PATH=$PATH:/rust/bin