Switch npm to Yarn (#1522)

* Add testUrl to jest config

* Switch npm to Yarn

dockers/Screenshotter/README.md

@@ -8,7 +8,7 @@ installed and running.
 If all you want is (re)create
 all the snapshots for all the browsers, then you can do so by:
 
-    npm run test:screenshots:update
+    yarn run test:screenshots:update
 
 It will fetch all required selenium docker images, and use them to
 take screenshots.
@@ -32,7 +32,7 @@ For Chrome, simply replace both occurrences of `firefox` with `chrome`.
 
 It is possible to run `screenshotter.js` without the use of Docker:
 
-    npm install selenium-webdriver
+    yarn add selenium-webdriver
     node dockers/Screenshotter/screenshotter.js
 
 This will generate screenshots using the Firefox installed on your system.
@@ -69,7 +69,7 @@ Examples:
 
 You can verify screenshots by running:
 
-    npm run test:screenshots
+    yarn run test:screenshots
 
 or passing `--verify` option to `screenshotter.js` or `screenshotter.sh`.
 See above for more details.

dockers/texcmp/README.md

@@ -30,7 +30,7 @@ the even larger overhead of setting up docker and creating the initial
 image, then you may instead execute the commands
 
     cd dockers/texcmp
-    npm install
+    yarn install
     node texcmp.js
 
 from the root of your KaTeX directory tree.  Required tools include the