Specify a type of DeploymentOptions.regions more precisely (firebase#507)

Author: merlinnot

src/function-configuration.ts

@@ -64,6 +64,6 @@ export interface RuntimeOptions {
 }
 
 export interface DeploymentOptions extends RuntimeOptions {
-  regions?: string[];
+  regions?: Array<typeof SUPPORTED_REGIONS[number]>;
   schedule?: Schedule;
 }