[FEATURE]Update doctrine/orm to 2.6

[sztyup]

Changes proposed in this pull request:

• Use the recently relased 2.6 version of the doctrine ORM which has many improvements.

[patrickbrouwers]

@sztyup how well did you test this in your own app/a test app? Does everything still work, no changes on doctrine side that break anything on our side?

[sztyup]

@patrickbrouwers I tested it on two of my own apps and as far as I can tell nothing breaks

[sztyup]

Here are the possible BC breaks from the update, the only important is the php7.1 requirement, which should to be documented here too.
EDIT: I realized its better than that, for users with 7.0 it sticks with orm 2.5 and for 7.1 it updates to 2.6
https://github.com/doctrine/doctrine2/releases/tag/v2.6.0

[xtrasmal]

Can we update?
I really like to use https://github.com/Ocramius/DoctrineBatchUtils which uses orm 2.6

[patrickbrouwers]

Thanks for the PR, installing with 2.6 should be possible now. Will indeed install 2.5 if you are using <7.1