build: fix error when running tslint task (#6431)

Fixes an error, that was being thrown when running the `tslint` task, by bumping `tslint` to 5.6.

package.json

@@ -113,7 +113,7 @@
     "stylelint": "^7.12.0",
     "ts-node": "^3.0.4",
     "tsconfig-paths": "^2.2.0",
-    "tslint": "~5.5.0",
+    "tslint": "~5.6.0",
     "tsutils": "^2.6.0",
     "typescript": "~2.2.1",
     "uglify-js": "^2.8.14",

src/lib/menu/menu-trigger.ts

@@ -31,8 +31,11 @@ import {
   OverlayRef,
   OverlayState,
   RepositionScrollStrategy,
-  ScrollStrategy,
   VerticalConnectionPos,
+  // This import is only used to define a generic type. The current TypeScript version incorrectly
+  // considers such imports as unused (https://github.com/Microsoft/TypeScript/issues/14953)
+  // tslint:disable-next-line:no-unused-variable
+  ScrollStrategy,
 } from '@angular/cdk/overlay';
 import {filter, RxChain} from '@angular/cdk/rxjs';
 import {MdMenu} from './menu-directive';