Add a travis build file.

It seems we have to run this on ubuntu trusty images to satisfy
the libopus-dev dependency, which currently requires sudo
(I think as proxy to trigger a non-docker image?) and is
therefore slower than vanilla container builds.

.travis.yml

+language: c
+
+# We need at least Ubuntu 14.04 for the libopus dependency.
+sudo: required
+dist: trusty
+
+addons:
+  apt:
+    packages:
+      - libogg-dev
+      - libopus-dev
+
+script:
+  - ./autogen.sh
+  - ./configure
+  - make
+  - make distcheck