Limit packaging extraneous files

[MarkHerhold]

matter.js is including a lot of extra files when published to npm. This PR removes them.

Here are the list of files that are currently being published:

   36.2 MiB [ 90.7%] /test                                                                                                                                                      
    2.7 MiB [  6.8%] /demo
  376.0 KiB [  0.9%] /build
  352.0 KiB [  0.9%] /src
  176.0 KiB [  0.4%] /examples
   44.0 KiB [  0.1%]  CHANGELOG.md
   12.0 KiB [  0.0%]  README.md
   12.0 KiB [  0.0%]  Gulpfile.js
    4.0 KiB [  0.0%]  package.json
    4.0 KiB [  0.0%]  .eslintrc
    4.0 KiB [  0.0%]  LICENSE
    4.0 KiB [  0.0%]  CONTRIBUTING.md
    4.0 KiB [  0.0%]  bower.json
    4.0 KiB [  0.0%]  .travis.yml
    4.0 KiB [  0.0%]  .npmignore

This PR removes things like CONTRIBUTING.md and .travis.yml.
package.json, the README, and the LICENSE are automatically retained by NPM.

https://docs.npmjs.com/files/package.json#files

[MarkHerhold]

@liabru Any chance this can get merged?

[liabru]

Looks good, this should be applied on the next release. Thanks guys.