Add Guard for specs
Updated README with a migration example for allowing nills

If you use the migration helper for an allow_nil field, it takes a bit of digging to work out how to do it. The normal approach of `null: false` doesn't work.

I've added a working example to the section on allowing nils to make it a bit clearer:

```ruby
add_money :products, :optional_price, amount: { null: true, default: nil }
```
bump money.gem requirement to 6.0
update contributors
version bump to 0.9.0
Merge pull request #122 from mcoms/patch-1

Updated README with a migration example for allowing nills
Merge pull request #144 from RubyMoney/dep-updates

update deps
Allow per-attribute disabling of validations
Merge pull request #146 from BrandyMint/guard

Add Guard for specs
Update .travis.yml
Merge pull request #145 from cbartlett/master

Allow per-attribute disabling of validations
Raise error when trying to change model based currency
Merge pull request #147 from BrandyMint/master

Raise error when trying to change model based currency
Fixed instance currency usage documentation
Merge pull request #152 from ntudor/documentation_update

Fixed instance currency usage documentation
Remove redundant logic in activerecord/monetizable
Update README with gem version and license badges
Silence Money deprecation warnings in tests
Ignore the dummy db files
Provide a rake task to prepare test environment