build: Add support for travis CI

It is useful to support a future staging branch and to have an
automated consistency check on github pull requests.

.travis.yml

+language: c
+sudo: false
+addons:
+  apt:
+    packages:
+      - yasm
+      - bc
+compiler:
+  - clang
+  - gcc
+cache:
+  directories:
+    - libav-samples
+script:
+  - mkdir -p libav-samples
+  - ./configure --samples=libav-samples --cc=$CC
+  - make -j 8
+  - make fate-rsync
+  - make check -j 8