Bump Travis Node to 12 (#961)

Summary:
Looks like jsdom for jest wants either 10 or 12, but not 11. ¯\_(ツ)_/¯
Pull Request resolved: https://github.com/facebook/flipper/pull/961

Test Plan: Watch Travis signal.

Reviewed By: cekkaewnumchai

Differential Revision: D20815326

Pulled By: passy

fbshipit-source-id: 6b7a8ddda75299443853d8bdca56fe1ce3d60afe

.travis.yml

@@ -6,7 +6,7 @@ matrix:
     - language: node_js
       os: linux
       node_js:
-        - "11"
+        - "12"
 
       install:
         - cd website
@@ -30,7 +30,7 @@ matrix:
     - language: node_js
       os: linux
       node_js:
-        - "11"
+        - "12"
 
       install:
         - cd desktop