getServicePort on DockerComposeContainer throws NullPointerException if service instance number in not used.

[barrycommins]

This is quite a minor one, but maybe worth fixing for consistency.

It is possible to expose a service in DockerComposeContainer without specifying the instance number:

new DockerComposeContainer(new File("src/test/resources/compose-test.yml"))
        .withExposedService("redis", 6379)

but when you then call getServicePort("redis", 6379), a NullPointerException is thrown.

It works ok with getServicePort("redis_1", 6379)

[bsideup]

@barrycommins nice! Was planning to do exactly that :D Thanks for contributing it :)

[kiview]

LGTM, I just deleted the corrupted Travis caches and restarted the build, should suceed now.

[rnorth]

Great! Thanks for this.
I'll merge.