fix(constants): Added pagination key constants

<!--- Provide a general summary of your changes in the Title above -->
Adding constants for our pagination methods. 

## Description
Used internally for a determining how to fetch the next page. 

fixes: EB-94542

<!--- Describe your changes in detail -->

<!--- Please include the phrase "BREAKING CHANGE:" here if your require a major release -->

<!--- Don't forget to note any issues here with "fixes #<issue number>" -->

## How Has This Been Tested?

Just constants so I didn't really test anything besides making sure `validate` passed. 

<!--- Please describe in detail how you tested your changes. -->

<!--- For bug fixes, include regression unit tests that fail without the fix -->

<!--- For new features, include unit tests for the new functionality -->

## Screenshots (if appropriate):

## Checklist:

<!--- Please mark an `x` in all the boxes that apply. -->

<!--- If you're unsure about any of these, don't hesitate to ask. We're here to help! -->

*   [x] I have read the [**CONTRIBUTING** document](https://github.com/eventbrite/eventbrite-sdk-javascript/blob/master/CONTRIBUTING.md).
*   [x] I have updated the documentation accordingly.
*   [x] I have added tests to cover my changes.
*   [x] I have run `yarn validate` to ensure that tests, typescript and linting are all in order.

.travis.yml

@@ -1,6 +1,7 @@
 language: node_js
 node_js:
-  - "node"
+# Removed until v11 bug is resolve see #43
+# - "node"
   - "10"
   - "8"
   - "6"

src/constants.ts

@@ -0,0 +1,2 @@
+export const PAGE_KEY = 'page';
+export const CONTINUATION_KEY = 'continuation';

src/index.ts

@@ -1,6 +1,8 @@
 import {Sdk, SdkConfig} from './types';
 import request from './request';
 
+export * from './constants';
+
 const DEFAULT_API_URL = 'https://www.eventbriteapi.com/v3';
 
 const eventbrite = ({