forgot to add webext as target

.travis.yml

@@ -3,9 +3,11 @@ env:
   matrix:
   - BROWSER=chromium EXT=zip
   - BROWSER=firefox EXT=xpi
+  - BROWSER=webext EXT=zip
 script: ./tools/make-${BROWSER}.sh all
 deploy:
   provider: releases
+  prerelease: true
   api_key:
     secure: eQgPAHH6PKu2dLK+NafxwLl66t0cyW5x5NZFquOwsNMal5nsfof7lyXj2F0Q0vUpGeI21MOipBI8UGv5oXPoiXnr0fhEbEBz65C9vypK61WkDCQVGVeZVNGQwSXUm6gD2EzpPgTCIs52+7dKCDJ3stXzdimOiOTYs4WMNKKarFM=
   file: dist/build/uMatrix.${BROWSER}.${EXT}