Remove class variable caching

* Simplify comments
Remove unused method #column_type_map
Spatial type is a Value

* Change base class from Type::String to Type::Value
* Move methods into OID::Spatial
* Add OID::Spatial#spatial?
* Fix SpatialColumn#spatial?
Update TableDefinition

* Update #column signature to match parent
* Remove unnecessary module scope
Add TableDefinition#spatial, #geography

* Remove unnecessary module scope
Update travis config
Remove unused test gemfiles