Use an env var for the package version

Because supplying arguments to npm scripts is a bad idea: it just
appends the arguments blindly to whatever the command was. Thanks,
npm.

scripts/package.sh

@@ -2,8 +2,8 @@
 
 set -e
 
-if [ $# -eq 1 ]; then
-    version=$1
+if [ -n "$DIST_VERSION" ]; then
+    version=$DIST_VERSION
 else
     version=`git describe --dirty --tags || echo unknown`
 fi