Added pullapprove config file

[ESapenaVentura]

Changelog

Added pullapprove config file with the following features:

• Whenever a pull request is opened, @lauraclarke @clairerye and @mcourtot will be notified.
• Pullapprove will wait until travis test passes
• Pullapprove will send a warning comment to the author if they try to push directly into master.

I haven't added it, but other possible interesting features we could add would be:

• PR review group: With a list of specific people that will be tagged as reviewers whenever a PR is requested. These people will be selected randomly from the list each time.
• Labels to skip pullapprove: Labels like "hotfix", "bugfix", etc won't trigger pullapprove

[ESapenaVentura]

On another note, @ke4 @rolando-ebi Where is the travis-test the travis config file refers to?

https://github.com/elixir-europe/json-schema-validator/blob/master/.travis.yml#L4

I can't see any travis status in the PRs, which seems pretty weird. This clarification is needed for this PR as one of the conditions for pullapprove to work (With this config) is for the travis build to be successful

[ke4]

@ESapenaVentura
Travis test defined here:

json-schema-validator/package.json

Line 11 in f9b761f

"travis-test": "jest test/elixir-validator.test.js"