add missing truncate comment to recent blog posts (#7655)

<!-- Thanks for submitting a pull request! Please provide enough information so that others can review your pull request. The two fields below are mandatory. -->

<!-- Please remember to update CHANGELOG.md in the root of the project if you have not done so. -->

## Summary

THey take unnecessary room currently in the lits: https://jestjs.io/blog/

<!-- Explain the **motivation** for making this change. What existing problem does the pull request solve? -->

## Test plan
Check netlify preview

<!-- Demonstrate the code is solid. Example: The exact commands you ran and their output, screenshots / videos if the pull request changes UI. -->

website/blog/2017-12-18-jest-22.md

@@ -7,6 +7,8 @@ authorFBID: 100003004880942
 
 Today we are announcing a new major version of Jest which refines almost all parts of Jest to provide a more solid testing foundation. Together with the Jest community we made a number of changes across the board that will help you get more out of Jest. We are also graduating the custom runners feature out of the experimental stage and added a new package, `jest-worker`, for parallelizing work across multiple processes. We have compiled a list of highlights below but make sure to check out the (as always) [massive changelog](https://github.com/facebook/jest/blob/master/CHANGELOG.md).
 
+<!--truncate-->
+
 ## Good bye Node 4 & welcome JSDOM 11
 
 With this release we are dropping support for Node 4, mainly because one of our main dependencies, JSDOM, ended their support. Jest now comes out of the box with JSDOM 11 which features better support for SVGs, `requestAnimationFrame`, `URL` and `URLSearchParams` built in, and [much more](https://github.com/tmpvar/jsdom/blob/master/Changelog.md).

website/blog/2018-05-29-jest-23-blazing-fast-delightful-testing.md

@@ -11,6 +11,8 @@ We would also like to welcome both [Babel](https://babeljs.io/) and [Webpack](ht
 
 Here's are some of the Jest 23 highlights and breaking changes.
 
+<!--truncate-->
+
 For a full list see the [changelog](https://github.com/facebook/jest/blob/master/CHANGELOG.md).
 
 ## Interactive Snapshot Mode

website/blog/2018-06-27-supporting-jest-open-source.md

@@ -15,6 +15,8 @@ At the Jest Summit we announced the Jest Open Collective. This is our way to hel
 
 In this post we'll outline what the Jest Open Collective is, the structure, and the goals we have.
 
+<!--truncate-->
+
 # The Jest Open Collective
 
 ![Banner image for Jest Open Collective](/img/blog/collective.png)