Add JSON Schema validation options to the gulp plugin options

[prantlf]

This is an example:

jsonlint({
    schema: {
        src: 'test/fixtures/schema.json',
        environment: 'json-schema-draft-04'
    }
})

This PR is based on #71.

[prantlf]

Travis build fails because of Node.js 6 incompatibility ineslint.