Fix apt-get crash in Travis CI for PHP 5.3

[mdupont]

Description

Fixes the crash when calling apt-get update in PHP 5.3 environment in Travis CI. Since the culprit is a 3rd party repo for MongoDB and that we don't need MongoDB, I just remove the repo definition so apt-get doesn't try to connect to it.

Fixes #1706

Checklist:

• I have run composer run-script check --timeout=0 and no errors were reported
• The new code is covered by unit tests (check build/coverage for coverage report)
• I have updated the documentation to describe the changes

[coveralls]

Coverage remained the same at 94.606% when pulling 7231176 on mdupont:fix-travis-ci into 85e9144 on PHPOffice:develop.

[mdupont]

Travis build worked as expected, but is still reported as failed because of phpmd warnings that were already existing in the code in the develop branch, nothing related to this PR.

[0b10011]

I bumped the dist to xenial in #1669 and everything passed without bumping memory or anything. Mind you, it also has already bumped the minimum to PHP 7.0 and code/tests has been fixed accordingly.

[mdupont]

If requirements get bumped to PHP >= 7.0 (and we can benefit from the enhanced type declarations), then indeed there won't be any issue anymore :-)

[stephanvierkant]

Wait, does this library still support PHP versions from the middle ages?