move the minimum required version of atoum to 2.6.0
move the minimum required version of atoum to 2.6.0

The improvements in https://github.com/atoum/autoloop-extension/pull/4
lies upon this PR on atoum : https://github.com/atoum/atoum/pull/567
which has been released on version 2.6.0.

atoum/autoloop-extension