complete rspecs for price model

[sawangupta92]

1. Use constant AMOUNT_LIMIT for maximum and minimum amount.
2. Write a shared example for paranoia named “a Paranoid model”
3. Write rspecs for Included/extended modules, Constants, Associations, Validations, Callbacks, Ransackable attributes, #money, #variant, #ensure_currency
4. Move #price_including_vat_for and #display_price_including_vat_for to describe block of Instance Methods
5. Require files of shared folder in spec_helper.rb
6. Updated rspecs for #price.

[jitendra]

@sawangupta92 Please provide details about each small change in the description of this PR.

[jitendra]

👍 please resolve conflict

[jitendra]

@sawangupta92 please raise on Spree with PR description