Use drush_path for update tasks

[mglaman]

For issue #36

[geerlingguy]

@mglaman - Can you rebase on master? The test fail is because of a change there.

Also, I see that I used {{ drush_path }} a couple other places, but technically provide no default (yikes!). It's added by the geerlingguy.drush role, but it might be good to fallback to default('drush') if no drush_path is set.

[mglaman]

@geerlingguy rebased. I'll admit, figuring out where to put default('drush') is above my head. I see {{ drush_path }} isused in build makefile as well.

[geerlingguy]

No problem, I'll create a follow-up, as this is kinda low priority, since it's highly likely if you use this role, you'll use geerlingguy.drush as well and it will never be a problem. Else, you can just set drush_path manually if needed.

[geerlingguy]

Created a follow-up issue #39