Release candidate build declaration fails for sandbox

[huntc]

sandbox run 1.1.8-rc.1 fails given that the - is interpreted as an option

[markusjura]

So far the sbt-conductr parser expects that the conductr version consists of only digits and dots: https://github.com/typesafehub/sbt-conductr/blob/master/src/main/scala/com/lightbend/conductr/sbt/ConductRPlugin.scala#L656

This is reason why it fails. We can relax the expected format. We need to be careful though to not generalize too much because otherwise the parser can't distinguish anymore between a conductr version and a option. This would mean that auto completion will not work anymore, because the conductr version can be put to any position.