Amend symfony/serializer dep to include version used in symfony 2.8 LTS

[nosnickid]

Symfony 2.8 is in LTS until 2019 ( http://symfony.com/doc/current/contributing/community/releases.html#schedule ), and projects based on it cannot use googleads-php-lib due to a dependency conflict.

This change permits the version included in 2.8. Unit tests pass with this version, and they seem to rely heavily on the serializer.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

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[nosnickid]

I signed it.

…

On 13 January 2017 at 13:07, googlebot ***@***.***> wrote: Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA). 📝 *Please visit https://cla.developers.google.com/ <https://cla.developers.google.com/> to sign.* Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks. ------------------------------ - If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data <https://cla.developers.google.com/clas> and verify that your email is set on your git commits <https://help.github.com/articles/setting-your-email-in-git/>. - If you signed the CLA as a corporation, please let us know the company's name. — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#217 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAJNMDkhDkVFPtBJT-wKXxxiOhIYRF2Bks5rR3cVgaJpZM4Li3Hc> .

[googlebot]

CLAs look good, thanks!

[nosnickid]

And, anecdotally speaking, this has worked fine for me in my afternoon's development on various AdGroup, Label, Campaign and BatchJob integrations.

[vtsao]

@nosnickid thanks for the PR and the update on testing this.

@fiboknacky can you do some more testing to ensure that ^2.8.0 of the serializer works with our lib before we accept this PR? It would be good to look at the breaking changes for the changelog of ^3.0.0 and see if anything affects us as well.

[fiboknacky]

It seems there are no breaking changes between ^2.8 and ^3.0 and both versions are pretty much the same.
I also tested with 2.8 real code and all related examples could be run. Unit tests also passed.
So, it makes sense to accept this pull request.

Thanks @nosnickid very much for the PR. :-)