meta: Set main.VersionPrerelease="dev"

For most projects using this versioning system, making a commit bumping
the version and re-adding the development tag is part of the release
SOP (certainly it was for Terraform!). I'm not sure why it didn't happen
here, but without this, all development builds show up as Nomad v0.6.0
with no dirty marker or commit marker.

version.go

@@ -15,7 +15,7 @@ const Version = "0.6.0"
 // A pre-release marker for the version. If this is "" (empty string)
 // then it means that it is a final release. Otherwise, this is a pre-release
 // such as "dev" (in development), "beta", "rc1", etc.
-const VersionPrerelease = ""
+const VersionPrerelease = "dev"
 
 // GetVersionParts returns the Nomad version strings. Printing of the Nomad
 // version should be used in conjunction with the PrettyVersion method.