Upgrade to eslint version 6

This major version bump required two changes:

- The global line in the mobile viewer example should be removed because
  the `.eslintrc` file already defines these globals and with the new
  `eslint` version we otherwise get an error saying "'pdfjsLib' is already
  defined as a built-in global variable".
- The ECMA version for the examples must be set to 6 since we're using
  modules, otherwise we get an error saying "sourceType 'module' is not
  supported when ecmaVersion < 2015". It turns out that the previous
  version of `eslint` already used ECMA version 6 silently even though
  we set 5, see eslint/eslint#9687 (comment),
  so in terms of our code nothing really changes.

examples/.eslintrc

@@ -4,7 +4,7 @@
   ],
 
   "parserOptions": {
-    "ecmaVersion": 5,
+    "ecmaVersion": 6,
   },
 
   "env": {

examples/mobile-viewer/viewer.js

@@ -12,7 +12,6 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-/* globals pdfjsLib, pdfjsViewer */
 
 'use strict';