Make silk optional (#24)

Author: blink1073

setup/setup.sh

@@ -43,7 +43,7 @@ AWS_BUCKET=${RELEASE_ASSETS_BUCKET:-}"
 GPG_KEY_ID=$GPG_KEY_ID
 GPG_PUBLIC_URL=${GPG_PUBLIC_URL:-}"
 GARASIGN_ENVFILE=$GARASIGN_ENVFILE
-SILKBOMB_ENVFILE=$SILKBOMB_ENVFILE
+SILKBOMB_ENVFILE=${SILKBOMB_ENVFILE:-}"
 ARTIFACTORY_REGISTRY=$ARTIFACTORY_REGISTRY
 RELEASE_ASSETS=$RELEASE_ASSETS
 S3_ASSETS=$S3_ASSETS