Adding hook_requirements() to commerce_price.install 

Adding hook_requirements() to commerce_price.install as uses the commerceguys/intl library which needs to be installed via composer first.
Was think that is someone tries to install all the commerce modules without having installed composer_manager, Drupal is going to ask do you want to install composer manager.

Not sure exactly when in that flow that hook_requirements() is called for each of the modules. Is it individually as it tries to install each of them or all of them at the start before trying to install any of them.

If we can guarantee that composer manager will have been installed through the module dependencies by the point we get to the next line then we don't need the if statement.