fix(lib/types/index.js): Replaced the gtfs field option type from value: syring to value : string |

I was getting a flow error of not being able to compare a string to an integer.

#656

Author: Robert Gregg

lib/types/index.js

@@ -578,7 +578,7 @@ export type GtfsSpecField = {
   helpContent?: string,
   inputType: InputType,
   name: string,
-  options?: Array<{text: string, value: string}>,
+  options?: Array<{text: string, value: string | number}>,
   placeholder?: string,
   required: boolean
 }