debian/control: bumping build-depends for debhelper

Since moving to debhelper as buildsystem we have to increase the
build depend on debhelper to version >= 7.0.50~ as we use the debhelper
override function. This is need at least debhelper >= 7.0.50.

This fixes Lintian warning
lintian: fixing debhelper-overrides-need-versioned-build-depends (>= 7.0.50~)