Fix NodeSeq quoting when toString is empty

adding `ivyXml` key brakes compilation if it's empty

that's an example of generated class
```
  val version = "0.1.1-SNAPSHOT"
  val scalaVersion = "2.9.2"
  val ivyXml =
  val ivyScala: Option[sbt.IvyScala] = Some("IvyScala(2.9.2,2.9.2,List(),true,true,true)")
  val ivyConfigurations = Seq("compile", "runtime", "test", "provided", "optional", "compile-internal", "runtime-internal", "test-internal", "plugin", "sources", "docs", "pom", "scripted-sbt")
```

this patch fixes it
I've also changed version to 0.2.6-SNAPSHOT 

please let me know if anything need to be adjusted
thanks