Bump .ruby-version to 2.1.5
Sync devtools
Nuke dead code
Rename Mom::Morpher::{Builder::Entity => Builder}
Nuke Mom::Entity in favor of optional Mom::Mapper

The Entity concept actually duplicated already available mapper
functionality with arguable little additional value. I even find myself
rarely using Mom::Mapper because I typically have no need to both load
and dump the same hash.
Simplify Definition::Registry#register
Return immutable registries from toplevel DSL
Nuke Entity concept leftovers
Turn Mom::Registry into an Enumerable
Adjust flay and flog scores
Simplify Environment::BUILD_OPTIONS
Introduce and use Definition::Registry#definition
Rename parameters in Environment
Move constant to where it semantically belongs
Fix whitespace
Avoid unnecessary calls to #dup
Move registry building to Environment
Nuke cache semantics from ad hoc morpher builders
Trim down toplevel morpher builder API
Refactor towards my current domain understanding