fix bugs in EnumValue

[NeoPhi]

TODO:

• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests pass
• Update CHANGELOG.md with your change
• If this was a change that affects the external API, update the README

[jnwng]

thanks @NeoPhi, just confirming that this is the best way to get this information for enums. im looking into the upstream issues with graphql-import that are showing up in travis

[NeoPhi]

Based on how https://github.com/graphql/graphql-js/blob/master/src/utilities/findDeprecatedUsages.js#L49 is implemented, I've not found another way to get access to getEnumValue given that https://github.com/graphql/graphql-js/blob/master/src/validation/validate.js#L97 doesn't expose that utility function nor does it export typeInfo.

[jnwng]

oh sorry @NeoPhi i had meant that i was confirming that "yes, this is the best way to do this". im figuring out what to do about upstream dependencies that don't match our engine support before merging / releasing this.

[jnwng]

finally — figured out what was going on in an upstream dependency that was being triggered here. turns out the Travis test runner uses npm instead of yarn, but in this case that was actually good as it revealed that we had dependencies that didn't work properly in Node v4/v5.