fix travis once more, this time due to jsdom breaking compatibility with nodeJS < 6

.travis.yml

@@ -15,7 +15,7 @@ before_script:
   - composer install -n
   - npm install -g mocha
   - cd js
-  - npm install jsverify jsdom jsdom-global
+  - npm install jsverify jsdom@9 jsdom-global@2
   - cd ..
 
 script:

tst/README.md

@@ -51,7 +51,7 @@ and jsdom-global locally:
 ```console
 $ npm install -g mocha istanbul
 $ cd PrivateBin/js
-$ npm install jsverify jsdom jsdom-global
+$ npm install jsverify jsdom@9 jsdom-global@2
 ```
 
 Example for Debian and Ubuntu, including steps to allow the current user to
@@ -63,9 +63,12 @@ $ sudo chown -R $(whoami) $(npm config get prefix)/{lib/node_modules,bin,share}
 $ ln -s /usr/bin/nodejs /usr/local/bin/node
 $ npm install -g mocha istanbul
 $ cd PrivateBin/js
-$ npm install jsverify jsdom jsdom-global
+$ npm install jsverify jsdom@9 jsdom-global@2
 ```
 
+Note: If you use a distribution that provides nodeJS >= 6, then you can install
+the latest jsdom and jsdom-global packages and don't need to use @9 and @2.
+
 To run the tests, just change into the `js` directory and run istanbul:
 ```console
 $ cd PrivateBin/js