Update to flow v0.38.0

[gabelevi]

I noticed some errors when running Flow v0.38.0 against a repo that used graphql-js. It looks like these two files were using [string], which is a tuple with a single string element, when they meant string[], which is an array of strings.

Flow v0.38.0 is stricter about tuples, so this no longer typechecked.

node_modules/graphql/validation/rules/ArgumentsOfCorrectType.js.flow:48
 48:               errors
                   ^^^^^^ array type. Only tuples and array literals with known elements can flow to
 22:   verboseErrors?: [string]
                       ^^^^^^^^ tuple type

node_modules/graphql/validation/rules/DefaultValuesOfCorrectType.js.flow:66
 66:               errors
                   ^^^^^^ array type. Only tuples and array literals with known elements can flow to
 33:   verboseErrors?: [string]
                       ^^^^^^^^ tuple type

server/fb-transforms/babel-relay-plugin/node_modules/graphql/validation/rules/ArgumentsOfCorrectType.js.flow:48
 48:               errors
                   ^^^^^^ array type. Only tuples and array literals with known elements can flow to
 22:   verboseErrors?: [string]
                       ^^^^^^^^ tuple type

server/fb-transforms/babel-relay-plugin/node_modules/graphql/validation/rules/DefaultValuesOfCorrectType.js.flow:66
 66:               errors
                   ^^^^^^ array type. Only tuples and array literals with known elements can flow to
 33:   verboseErrors?: [string]
                       ^^^^^^^^ tuple type

[gabelevi]

It would be great if this change could be published as soon as possible. Anyone using Flow >v0.38.0 with graphql-js will get those 4 errors.

As for yarn, I did the following

yarn install --pure-lockfile # avoid messing with yarn.lock more than necessary
yarn add --dev flow-bin@0.38.0
./node_modules/.bin/flow check # no errors

[gabelevi]

If this is merged then #670 is no longer necessary

[robzhu]

Thanks!