Allow interfaces to have no implementors (graphql#1376)

* Allow interfaces to have no implementors
Reverts graphql#1277, and implements spec removal from graphql#459. While it's an open question whether this validation is good, we need to provide an upgrade path for schemas that currently do not satisfy this validation rule.

* Update test to accept unimplemented interface

14.0.0-rc.1 (graphql#1364)

If you're a third-party library used by others who may also use `graphql-js`, I do not recommend take a dependency on this RC: there will be a few additional breaking changes, such as a fix for graphql#1324 .

0.13.2

0.13.1

0.13.0

0.13.0-rc.1

0.12.3

0.12.2

0.12.1

0.12.0