fix(package): update bootstrap to version 4.2.1

Closes #36

Author: greenkeeper[bot]

package.json

@@ -21,7 +21,7 @@
     "@angular/platform-browser": "^5.0.2",
     "@angular/platform-browser-dynamic": "^5.0.2",
     "@angular/router": "^5.0.2",
-    "bootstrap": "^3.3.7",
+    "bootstrap": "^4.2.1",
     "core-js": "^2.4.1",
     "rxjs": "^5.4.1",
     "zone.js": "^0.8.14"