Lock monetize version to 1.8.x
Lock monetize version to 1.8.x

This commit comes from @huoxito work on nebulab/solidus#15 (Thanks!)

On newer versions monetize requires money 6.12 which has a rewrite of
the Money#allocate method that we use in DistrubutedAmount calculator.

This is the PR that is causing issues: RubyMoney/money#772
This is the issue we need to be solved before relaxing the dependency
again: RubyMoney/monetize#118

pervino/solidus,pervino/solidus,pervino/solidus,pervino/solidus