Improve the gql tagged template literal types

The `gql` types were originally taken from
https://github.com/apollographql/graphql-tag/blob/50a850e484a60d95ddb99801c39785031e55b7a2/index.d.ts,
but those don't seem to be quite right, since the first argument
of a tagged template function is always an array of strings. Adjusting
to be more specific (thanks for catching this @benjamn).

src/types/graphql-tag.d.ts

@@ -5,7 +5,7 @@
 // be fully updated to use Typescript, and these discrepancies will be fixed.
 
 declare module 'graphql-tag' {
-  function gql(literals: any, ...placeholders: any[]): any;
+  function gql(literals: string[], ...placeholders: any[]): any;
   namespace gql {
     export function resetCaches(): void;
     export function disableFragmentWarnings(): void;