Fix `flow` (and `npm run lint`)

The npm library promise (https://github.com/then/promise) was updated
(roughly 2 hours ago) and our flow setup does not pass successfully on
it.

.flowconfig

@@ -19,6 +19,7 @@
 .*/node_modules/babel-core/.*
 .*/node_modules/bower/.*
 .*/node_modules/jsonlint/.*
+.*/node_modules/promise/index.js.flow
 .*/node_modules/styled-components/.*
 
 [include]