Add tests on armhf and powerpc linux on Travis-CI

"powerpc" is big endian, all other tested architectures are little endian.

.travis.yml

@@ -29,6 +29,20 @@ matrix:
     - os: osx
       rvm: 2.4.2
   include:
+    - name: powerpc
+      language: generic
+      before_install: |
+        docker run --rm --privileged multiarch/qemu-user-static:register --reset &&
+          docker build --rm -t ffi-powerpc -f spec/env/Dockerfile.powerpc .
+      script: |
+        docker run --rm -t -v `pwd`:/ffi ffi-powerpc
+    - name: armhf
+      language: generic
+      before_install: |
+        docker run --rm --privileged multiarch/qemu-user-static:register --reset &&
+          docker build --rm -t ffi-armhf -f spec/env/Dockerfile.armhf .
+      script: |
+        docker run --rm -t -v `pwd`:/ffi ffi-armhf
     - os: osx
       osx_image: xcode9.1
       rvm: 2.4.2

spec/env/Dockerfile.armhf

@@ -0,0 +1,25 @@
+# See .travis.yml how this docker image can be used.
+FROM multiarch/ubuntu-debootstrap:armhf-bionic
+
+RUN uname -a
+RUN apt-get update -qq && \
+  apt-get install -yq \
+    autoconf \
+    automake \
+    file \
+    gcc \
+    git \
+    libtool \
+    make \
+    ruby-dev
+RUN ruby --version
+
+WORKDIR /ffi
+COPY . .
+
+RUN gem install bundler --no-doc && \
+    bundle install
+
+CMD bundle install && \
+    bundle exec rake compile && \
+    bundle exec rake test

spec/env/Dockerfile.powerpc

@@ -0,0 +1,25 @@
+# See .travis.yml how this docker image can be used.
+FROM multiarch/ubuntu-debootstrap:powerpc-xenial
+
+RUN uname -a
+RUN apt-get update -qq && \
+  apt-get install -yq \
+    autoconf \
+    automake \
+    file \
+    gcc \
+    git \
+    libtool \
+    make \
+    ruby-dev
+RUN ruby --version
+
+WORKDIR /ffi
+COPY . .
+
+RUN gem install bundler --no-doc && \
+    bundle install
+
+CMD bundle install && \
+    bundle exec rake compile && \
+    bundle exec rake test