Use locally-installed Gulp over globally-installed one

Contributing.md

@@ -15,9 +15,8 @@ Below is an overview of how to contribute code to the PDF.js project. The basic
 ### Prerequisites
 * Git client
 * GitHub account
-* [Node.js](https://nodejs.org/)
-* Gulp (via `npm install -g gulp-cli`)
-* An ES6 compatible browser
+* [Node.js](https://nodejs.org/) with NPM
+* Modern browser
 
 If you develop on Windows, read [[Setting up pdf.js Development Environment for Windows]]. For font testing you will need [Python](https://www.python.org/download).
 
@@ -64,13 +63,13 @@ If the branch contains lot of small commits, you might be asked to squash the co
 To make sure that your code follows our [[Style Guide]], run the following command from the PDF.js folder:
 
 ```
-gulp lint
+npx gulp lint
 ```
 
 If there are errors, some of them can be fixed automatically by the linting tool. To do so, run the following command:
 
 ```
-gulp lint --fix
+npx gulp lint --fix
 ```
 
 _Protip_: If you are a Vim user, then install [Syntastic](https://www.vim.org/scripts/script.php?script_id=2736), install ESLint globally using `npm install -g eslint` and add the following line to your `.vimrc`:
@@ -102,13 +101,13 @@ The browser tests are split into the following types of tests:
 The reference tests require you to generate original snapshots for comparison. The snapshots should be generated before you make any changes. If you have already made some changes, either `git stash` your work or checkout the master branch. Now we can generate the reference images:
 
 ```
-gulp makeref
+npx gulp makeref
 ```
 
 You can then run the test suite from the PDF.js root folder after unstashing any changes or going back to your feature branch:
 
 ```
-gulp test
+npx gulp test
 ```
 
 If you wish to add a reference test for your patch, you can place the file (`foo.pdf` in this example) in the `test/pdfs` directory and run the following script from the PDF.js root folder to add it to the suite:
@@ -121,10 +120,10 @@ Note that this script only supports adding `eq` tests for all pages of the PDF f
 
 **Running unit tests separately**
 
-Unit tests are run when `gulp test` is run, but they can also be run separately using:
+Unit tests are run when `npx gulp test` is run, but they can also be run separately using:
 
 ```
-gulp unittest
+npx gulp unittest
 ```
 
 ### 5. Push changes to your fork/branch