criu: add .travis.yml (v3)

"""
Travis CI is configured by adding a file named .travis.yml, which is a
YAML format text file, to the root directory of the GitHub
repository.[5]

Travis CI automatically detects when a commit has been made and pushed
to a GitHub repository that is using Travis CI, and each time this
happens, it will try to build the project and run tests.
""" https://en.wikipedia.org/wiki/Travis_CI

Currently Travis CI builds criu for x86_64 and ARM

v2: move travis-ci.sh in scripts
v3: fix path to the script in the script