fix(UnionTypeComposer): add support for graphql@0.13.0

Closes #167

src/UnionTypeComposer.js

@@ -13,7 +13,7 @@ import type { Thunk } from './utils/definitions';
 import { resolveTypeArrayAsThunk } from './utils/configAsThunk';
 // import { typeByPath } from './utils/typeByPath';
 import { getGraphQLType, getComposeTypeName } from './utils/typeHelpers';
-// import { graphqlVersion } from './utils/graphqlVersion';
+import { graphqlVersion } from './utils/graphqlVersion';
 
 export type GraphQLUnionTypeExtended<TSource, TContext> = GraphQLUnionType & {
   _gqcTypeMap?: Map<string, ComposeObjectType>,
@@ -140,9 +140,16 @@ export class UnionTypeComposer<TContext> {
       });
       this.gqType._gqcTypeMap = m;
 
-      this.gqType._types = () => {
-        return resolveTypeArrayAsThunk(this.schemaComposer, this.getTypes(), this.getTypeName());
-      };
+      if (graphqlVersion >= 14) {
+        this.gqType._types = () => {
+          return resolveTypeArrayAsThunk(this.schemaComposer, this.getTypes(), this.getTypeName());
+        };
+      } else {
+        (this.gqType: any)._types = null;
+        (this.gqType: any)._typeConfig.types = () => {
+          return resolveTypeArrayAsThunk(this.schemaComposer, this.getTypes(), this.getTypeName());
+        };
+      }
     }
 
     return this.gqType._gqcTypeMap;