[HttpKernel] Fix event dispatcher dependency

Update composer.json to account for #9792
[HttpKernel] added value of DateTime object and objects implementing DateTimeInterface to ValueExporter
Merge branch '2.5'

* 2.5:
  PHP Fatal error when getContainer method of ContainerAwareCommand has be...
  [HttpFoundation] Fixed isSecure() check to be compliant with the docs
  Update MimeTypeExtensionGuesser.php
  fix test src/Symfony/Component/HttpFoundation/Tests/BinaryFileResponseTest.php
  [Process] Do not redirect output to file handles when output is disabled
  [Validator] Fix array notation in the PropertyPath::append()
  Fixed undefined ImageValidator:: property when uploading an image during functional tests
  [HttpKernel] Fix event dispatcher dependency
  Fixed the Travis build on PHP 5.3.3
feature #11070 [HttpKernel] added value of DateTime object to ValueExporter (scuben)

This PR was merged into the 2.6-dev branch.

Discussion
----------

[HttpKernel] added value of DateTime object to ValueExporter

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

This will output the actual value of a DateTime object in the form panel of the web debug toolbar.

- [x] gather feedback for my change (thank you stof, its my first contribution to oss)
- [x] finish the code

Commits
-------

a1762fb [HttpKernel] added value of DateTime object and objects implementing DateTimeInterface to ValueExporter