Switch to 2.1.1 in ruby-version
Remove obsolete methods from Mapper::Builder::Definition
Decouple mapper definition from axiom
Rename Mapper::Builder => Relation::MapperBuilder

This clearly indicates that it's part of relation as we're building
mappers using information *from schema* (currently keys and types)

Additionally remove :name from Mapper::Definition to lower the coupling
Expose mapper builder DSL in Mapper.build too (because why not)