[Travis] Add swap to avoid hitting memory issues

[mnocon]

Question	Answer
JIRA issue	n/a
Type	Misc
Target version	1.7 - same as #96
BC breaks	no
Doc needed	no

All PHP 5.6 jobs are failing on Travis because of memory issues, see: https://travis-ci.org/ezsystems/ezplatform-xmltext-fieldtype/builds/566005333?utm_source=github_status&utm_medium=notification

I suppose we could try limiting the minimum supported versions in requirements in this bundle:

    "require": {
        "ezsystems/ezpublish-kernel": "^6.11@dev || ^7.0@dev",
        "ezsystems/repository-forms": "^1.9 || ^2.0"
    },

but as I don't know with which version is the 1.7 line intended to be used. To avoid preventing users from installing it I've decided to simply add swap for Travis, as described in https://getcomposer.org/doc/articles/troubleshooting.md#proc-open-fork-failed-errors

I've tested the build with 1024 and it still failed, hence 2048 here.

TODO:

• Implement feature / fix a bug.
• Ask for Code Review.

[andrerom]

Damn, so we need this on almost every repo dealing with PHP 5 then?

Note that Composer 1.9.0 is out as well which supposedly uses a bit less memory in some cases:
composer/composer#8200

[mnocon]

Damn, so we need this on almost every repo dealing with PHP 5 then?

I hope not, the kernel 5.6 jobs are still looking ok (example: https://travis-ci.org/ezsystems/ezpublish-kernel/jobs/567655451)

I think in this repo we have the unlucky combination of both PHP 5.6 and a broad set of requirements (support for Kernel 6.11 etc.). In kernel we limit the requirements from time to time (ezsystems/ezpublish-kernel#2613 for example), not sure if we can/should do that in this package.