add travis config to run unit and integration tests.

[abacaphiliac]

this is an extension of #46

using travis i'm able to build minimum and maximum composer dependency versions, and run integration tests by installing the appropriate version of apc/apcu and running redis via docker service.

as i mentioned before, i hope this request doesn't come across too pushy. i am simply more familiar with travis than circle.

• add travis config to test PHP versions 5.5 to 7.1.
• each PHP version is tested with the minimum and maximum dependency versions which provides some certainty that our dependency ranges in the composer config are valid.
• update feature-generator in integration-test to return json-object with all required keys and appropriate variation-structures.
• install appropriate apc/apcu extension version based on php version.
• add apcu-feature-requester which extends the apc-feature-requester and uses the actively-maintained extension.
• add missing monolog requirement to integration-test composer config.
• run integration tests in travis.

sample build output, demonstrating that unit and integration tests pass on php55 through php71 with min and max composer dependency versions:
https://travis-ci.org/abacaphiliac/php-client/builds/184885080

[drichelson]

@abacaphiliac Thanks for the PRs! No comments on the contributions yet, but I just wanted you to know we will be taking a look.

[abacaphiliac]

@drichelson thanks! i have a plan (and a commit) to add Redis Sentinel support but i'd prefer to see the update pass integration tests in CI for all supported PHP versions, if possible.

[drichelson]

@abacaphiliac Thanks to your contributions I think we have equivalent testing now in CircleCI which is our primary CI system. Do you mind if I close this for now?

[abacaphiliac]

nah, go for it.

i will say that this merge request demonstrates successful execution of integration tests in TravisCI, which i think we haven't quite achieved in CircleCI. but i think we're getting close.