Use a doctrine/migrations version compatible with PHP 5.6 and 7.0

Starting in the 1.5 series, doctrine/migrations allows using a version
of ocramius/package-versions that depends on PHP 7.0 and up (usage of
return type hints, scalar type hints, and, in even later versions,
void return types).

This patch installs dependencies using PHP 5.6, to allow testing across
all versions.

Author: weierophinney