* Use mock canvas object replacing `canvas`

Fixes #3515.

A mock canvas object is added to the unit tests, which makes usage of module `canvas` voluntary.

The issue with `canvas` is that it requires an external dependency to `cairo`. This complicates setting up a develop environment for `vis.js`

- Removed `canvas` from `package.json`
- Added section to README.md with instructions on how to install `canvas` instead.

* Removed debugger statements

* Updates for review

* Fixes for review

* Enable linting for Travis

Since all code has been linted (thanks **TODO**!), it is now safe to enable linting for Travis.

Some cleanup actions have been performed; notably the additional `gulp` tasks for modules has
been removed. Since all code has been fixed for linting, these are not useful any more.

* Force linting error to check travis working

* Travis works! Remove forced linting error

* Updated babel modules to latest stable

* Updated devDependencies all modules expect webpack

* Updated webpack to latest stable

* First working version of eslint enabled with gulp

* Tryout of eslint with graph3d files

* Cleanup of gulp and eslint files.

* Completed test by linting graph3d

* Disable travis linting for now

* Remove global cmd param from .travis.yml

* Add C++11 and canvas module for travis tests

* Added libgif, updated version gulp-clean-css

* Update version webpack

* Force versions minimatch, graceful-fs; fixes for upgraded webpack

* Force version minimatch through travis.yml

* Fix comma's in json

* Add extraneous modules to package.json; final attempt at forcing versions of minimatch and graceful-fs

* Final changes module versions

* Fix due to linting

* Fix typo in package.json

* Upgrade eslint

* Fix redraw order

* Fix error when option is not defined

* Allow template labels

* Add .travis.yml file

* Add experiment travis code

* Fix react example

* created a checklist for the release process

* unchecked everything

* added make github release

* Remove npm-debug logs

* Release v4.20.1

* Update HISTORY.md

Fix indents

* Update HISTORY.md

* Update HISTORY.md

move - FEAT #3154 to timeline

* Release v4.20.0
* added new pull requests to the history
* fixed release date

see https://github.com/moment/moment/pull/3853

fixes #2912

fixes #2685

fixes #2784

Somewhat more complicated setup, associated with the need for a real window.

fixes #2685

If you want to integrate the vis.js sources in a project you need some babel-pugins.

see #2353

Fixes default end, and reduces duplicated code. Adds tests.

* updated HISTORY.md for v4.18.0
* bumped package version to 4.18.0

* address IE11 issue 'Symbol' is undefined with babel-polyfill  

https://github.com/almende/vis/issues/2560

* address IE11 issue 'Symbol' is undefined with babel-polyfill

https://github.com/almende/vis/issues/2560

* address IE11 issue 'Symbol' is undefined with babel-polyfill

https://github.com/almende/vis/issues/2560

* move all babel options to .babelrc file

* updated babel-loader

updated how_to_puplish

* replaced deprecated gulp-minify-css with gulp-clean-css
* updated mocha

`vis-network.min.js`, and `vis-timeline-graph2d.min.js`