travis: Don't install ccache in containers

We no longer use it, we use sccache

src/ci/docker/arm-android/Dockerfile

@@ -11,7 +11,6 @@ RUN dpkg --add-architecture i386 && \
   python2.7 \
   git \
   cmake \
-  ccache \
   unzip \
   expect \
   openjdk-9-jre \

src/ci/docker/cross/Dockerfile

@@ -9,7 +9,6 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   python2.7 \
   git \
   cmake \
-  ccache \
   sudo \
   gcc-aarch64-linux-gnu libc6-dev-arm64-cross \
   gcc-arm-linux-gnueabi libc6-dev-armel-cross \

src/ci/docker/i686-gnu-nopt/Dockerfile

@@ -9,7 +9,6 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   python2.7 \
   git \
   cmake \
-  ccache \
   sudo \
   gdb \
   xz-utils

src/ci/docker/i686-gnu/Dockerfile

@@ -9,7 +9,6 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   python2.7 \
   git \
   cmake \
-  ccache \
   sudo \
   gdb \
   xz-utils

src/ci/docker/x86_64-freebsd/Dockerfile

@@ -9,7 +9,6 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   python2.7 \
   git \
   cmake \
-  ccache \
   sudo \
   bzip2 \
   xz-utils \

src/ci/docker/x86_64-gnu-cargotest/Dockerfile

@@ -9,7 +9,6 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   python2.7 \
   git \
   cmake \
-  ccache \
   libssl-dev \
   sudo \
   xz-utils \

src/ci/docker/x86_64-gnu-debug/Dockerfile

@@ -9,7 +9,6 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   python2.7 \
   git \
   cmake \
-  ccache \
   sudo \
   gdb \
   xz-utils

src/ci/docker/x86_64-gnu-llvm-3.7/Dockerfile

@@ -9,7 +9,6 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   python2.7 \
   git \
   cmake \
-  ccache \
   sudo \
   gdb \
   llvm-3.7-tools \

src/ci/docker/x86_64-gnu-make/Dockerfile

@@ -9,7 +9,6 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   python2.7 \
   git \
   cmake \
-  ccache \
   sudo \
   gdb \
   xz-utils

src/ci/docker/x86_64-gnu-nopt/Dockerfile

@@ -9,7 +9,6 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   python2.7 \
   git \
   cmake \
-  ccache \
   sudo \
   gdb \
   xz-utils

src/ci/docker/x86_64-gnu/Dockerfile

@@ -9,7 +9,6 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   python2.7 \
   git \
   cmake \
-  ccache \
   sudo \
   gdb \
   xz-utils

src/ci/docker/x86_64-musl/Dockerfile

@@ -9,7 +9,6 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   python2.7 \
   git \
   cmake \
-  ccache \
   xz-utils \
   sudo \
   gdb