pin jszip to 3.3.0 to try to fix appveyor build

jszip 3.4.0 apparently has typescript definitions which may cause conflicts

related to discussion in #706

package.json

@@ -63,7 +63,7 @@
     "@types/vexflow": "^1.2.33",
     "es6-promise": "^4.2.5",
     "jspdf-yworks": "^2.1.1",
-    "jszip": "^3.0.0",
+    "jszip": "3.3.0",
     "loglevel": "^1.6.8",
     "svg2pdf.js": "^1.5.0",
     "typescript-collections": "^1.3.3",