Golint: Refactor config keys

[lucasgomide]

lint suggested

lucasgomide/snitch/hook/newrelic.go:16:2: struct field ApplicationId should be ApplicationID
lucasgomide/snitch/hook/newrelic.go:17:2: struct field ApiKey should be APIKey
lucasgomide/snitch/hook/slack.go:14:2: struct field WebhookUrl should be WebhookURL
lucasgomide/snitch/hook/slack_test.go:9:5: var webhookUrl should be webhookURL

In order to fix it, we need refactor func executeHook(h types.Hook, deploy []types.Deploy, conf interface{}) to don't use reflect