Add missing calculator method stub

We don't know how to handle shipping yet.  But prevent blow-ups as long as shipping is detected to be 0.
Calculate tax earlier

Front end needs to know taxes before it submits "payments".
Don't recalculate tax on an order when an address is changed and it is completed already
remove temporary workaround

spree now has a published version number (2.2.2) that solves the problem.
Merge branch 'remove-workaround' into 2-2-stable
minor reformat

move "def self.X" into "class << self..." block. preparation for adding more class methods.
generate new tax adjustments in closed state

prep work for subsequent closed-tax changes
update all past tax adjustments to a closed state

more prep work subsequent closed-tax changes
Update tax adjustments to always be closed

This prevents Spree from trying to automatically recalculate taxes on its own.
We have our own flow for figuring out when taxes should be recalculated and don't want Spree doing its own thing.
This should make things more robust and also lets us clean up some messy code in our pseudo-tax-calculater (spree/calculator/avatax.rb).
Note that there is a new migration here that needs to be run.

Also update the README to list some known issues.
Delete stale tax adjustments, even when their line items are missing

This shouldn't happen, but we currently do have some adjustments whose
line items have disappeared. This will include getting rid of those,
also.

This will fix
https://trello.com/c/soEC8z4P/49-spree-backend-error-nomethoderror-undefined-method-persisted-for-nil-nilclass
and
https://www.honeybadger.io/inspector/p/33720/5895315/22#comment_1471956
Delete all tax adjustments before recalculating
Add readme note about returns/refunds/etc.
Attempt to fix TravisCI integration

Reading the TravisCI docs it seems like 2.1.0 is not pre-installed on the workers so falling back to .ruby-version is not enough.
Add build status image to README
Move Spree::Calculator::Avatax to SpreeAvatax::Calculator

Use our own namespace instead of putting stuff in Spree's namespace.

NOTE:  This requires updating the end application's configuration as per the Readme. As well as any other place the end application might be referencing Spree::Calculator::Avatax.
add migration for class name change
Updates for spec deprecations

silence some spec deprecations noise
Gemfile debugger update

To work with Ruby 2.0+
Remove unused constant
merging bonobos upstream with my work to stay up to date