Adds Exclusions to dependency API

[juanpedromoreno]

This PR allows to optionally exclude transitive dependencies in code evaluations. For example:

curl -X "POST" "http://localhost:8080/eval" \
     -H 'Content-Type: application/json' \
     -H 'x-scala-eval-api-token: eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eW91ciBpZGVudGl0eQ.cfH43Wa7k_w1i0W2pQhV1k21t2JqER9lw5EpJcENRMI' \
     -d $'{
  "code": "{import stdlib._;import stdlib.Asserts._;import org.scalatest._;def a(n: Int) = n*2;a(1) should be(2)}",
  "resolvers": [
    "https://oss.sonatype.org/content/repositories/snapshots",
    "https://oss.sonatype.org/content/repositories/releases"
  ],
  "dependencies": [
    {
      "groupId": "org.scala-exercises",
      "exclusions": [
        {
          "organization": "io.monix",
          "moduleName": "monix_2.11"
        }
      ],
      "artifactId": "exercises-stdlib_2.11",
      "version": "0.5.0-SNAPSHOT"
    }
  ]
}'