fix: upgrade rxjs from 5.5.6 to 5.5.12

Snyk has created this PR to upgrade rxjs from 5.5.6 to 5.5.12.

See this package in NPM:
https://www.npmjs.com/package/rxjs

See this project in Snyk:
https://app.snyk.io/org/marcusasplund/project/475ed275-e380-4f99-b9c1-3d111522f528?utm_source=github&utm_medium=upgrade-pr

Author: snyk-bot

package.json

@@ -32,7 +32,7 @@
     "ionic-angular": "3.9.2",
     "ionic-plugin-keyboard": "2.2.1",
     "ionicons": "3.0.0",
-    "rxjs": "5.5.6",
+    "rxjs": "5.5.12",
     "sw-toolbox": "3.6.0",
     "zone.js": "0.10.3"
   },