chore(travis): bumps node versions used in Travis CI to 10 and 12 (#2300

)

Summary:
**Summary**

As of January 2020, Node 8 is past Active LTS end of life. This PR removes Node 8 from the Travis CI environment config and replaces it with Node 12.

https://nodejs.org/en/about/releases/

**Test Plan**

Tested locally building all targets on Node 12 with [nvm](http://nvm.sh/)
```
nvm use 12
yarn
```
Pull Request resolved: #2300

Reviewed By: mrkev

Differential Revision: D19298082

Pulled By: claudiopro

fbshipit-source-id: 0ce83534231f1b881b2f56e0410e99319d0f2365

.travis.yml

@@ -1,8 +1,8 @@
 language: node_js
 
 node_js:
-- 8
 - 10
+- 12
 
 sudo: false
 

package.json

@@ -85,7 +85,7 @@
     "webpack-stream": "^4.0.0"
   },
   "devEngines": {
-    "node": "8.x || 9.x || 10.x",
+    "node": "10.x || 12.x || 13.x",
     "npm": "2.x || 3.x || 5.x || 6.x"
   },
   "jest": {