fix(version): make the version numbers const

simple fix, thanks to @fmilo in #coreos

scripts/release-version

@@ -4,5 +4,5 @@ VER=$(git describe --tags HEAD)
 
 cat <<EOF
 package main
-var releaseVersion = "$VER"
+const releaseVersion = "$VER"
 EOF

version.go

@@ -1,3 +1,3 @@
 package main
 
-var version = "v1"
+const version = "v1"