Use travis_wait to avoid travis timeout on `composer up`
Jump to `memory_limit -1` for `composer up`
Also, since composer.lock is ignored, we can remove it.
Having a big composer.lock on a final project can have side effect on incoming PR that add a new vendor.
Mostly because conflict are too frequent.
By ignoring composer.lock we ease the PR submission and rebase.
BUT we need to be careful when we release a new version of wallabag. We should manually `git add -f composer.lock` to update it.
Since composer.lock will no longer be commited I switch the `composer install` to a `composer up` in the travis configuration.
