build: add Travis config for continuous integration

Run configure, build and run the tests. Pass the compiler -Werror to
confirm that the build does not produce any warnings. In case test cases
fail, include the failure log in the console output.

Run "make distcheck" to confirm that creating an archive works as
expected.

NOT YET: Build it using both "gcc" and "clang" compilers to confirm that
the build works on both.

Signed-off-by: Filipe Brandenburger <filbranden@google.com>