travis: build using devkitpro docker image

This is actually supported and robust, as oposed to downloading
these packages from the CI builders.

.ci/Dockerfile

@@ -0,0 +1,6 @@
+FROM devkitpro/devkitarm
+
+RUN apt-get update && \
+    apt install -y gcc && \
+    apt-get clean && \
+    rm -rf /var/lib/apt/lists/*

.travis.yml

@@ -1,15 +1,15 @@
 language: c
 
+sudo: required
+
+services: docker
+
 install:
-  - wget https://github.com/devkitPro/pacman/releases/download/devkitpro-pacman-1.0.1/devkitpro-pacman.deb &&
-    sudo dpkg -i devkitpro-pacman.deb
-  - sudo dkp-pacman --noconfirm -S gba-dev
-  - export DEVKITPRO=/opt/devkitpro
+  - docker build -t build .ci/
   - pip install --user cpp-coveralls
 
 script:
-  - make all examples
-  - make check
+  - docker run -v $TRAVIS_BUILD_DIR:/build build make -C /build all examples check
 
 after_success:
   - coveralls --include src --gcov-options '\-lp'