Update travis config for Erlang versions

- Add R16B02

- Remove some older releases from testing. I don't think there's
  anything that doesn't work there, but testing latest patch of a
  release is enough.
Consolidate type specs for keys to ej_key_path() type

Now that lists and tuples are supported, it makes sense to have a type
for key paths to simplify specs for get/set/delete.

Also fixed missing part of spec for key_type() to include the array
matching `{select, V}` stuff.
Enhance docs for get and set_p

Also cleaned up trailing whitespace.