fix Ldap converter serialize test

Starting from PHP versions 5.4.30, 5.5.14 and 5.6.0beta4, the serialized
representation of a DateTime includes fractional seconds.
The data provider must check the PHP version and provide a correct
serialized representation of the DateTime object.

refs #6468
fix Atom tests with fractional seconds in dates

the DateTime object is correctly created in Extension\Atom\Entry#L173,
but the tests created the comparison DateTime with the wrong format (ISO8601)
and the wrong datetime string (did not include the fractional seconds)
remove conditional in dataProvider and replace with serialized object
Merge pull request #6610 from gkralik/fix/travis-failing-tests

Fix travis failing tests
Merge branch 'hotfix/6610'

Close #6610