Bump to vertx 2.1M4-SNAPSHOT

2.1M3-SNAPSHOT is gone from maven-snapshot, update to the latest one.
Fixes the following `mvn install` error:

    [ERROR] Failed to execute goal on project nodyn-verticle: Could not
    resolve dependencies for project
    org.projectodd:nodyn-verticle:jar:0.1.1-SNAPSHOT: The following
    artifacts could not be resolved:
    io.vertx:vertx-core:jar:2.1M3-SNAPSHOT,
    io.vertx:vertx-platform:jar:2.1M3-SNAPSHOT: Could not find artifact
    io.vertx:vertx-core:jar:2.1M3-SNAPSHOT in projectodd-snapshot
    (https://repository-projectodd.forge.cloudbees.com/snapshot) ->
    [Help 1]
Merge pull request #38 from bnoordhuis/update-vertx-dependency

Bump to vertx 2.1M4-SNAPSHOT