Setup travis CI testing (#58)

Observe that it passed on this commit, but failed builds [#2: Break Tests][2], [#3: Break Flow][3], and [#4: Break Prettier][4]. 

Close #23 

[2]: https://travis-ci.org/sourcecred/sourcecred/builds/348453195
[3]: https://travis-ci.org/sourcecred/sourcecred/builds/348454983
[4]: https://travis-ci.org/sourcecred/sourcecred/builds/348455387

.travis.yml

@@ -0,0 +1,4 @@
+language: node_js
+node_js:
+  - "node"
+script: yarn travis

package.json

@@ -52,10 +52,12 @@
     "precommit": "npm run lint-staged",
     "lint-staged": "lint-staged",
     "prettify": "prettier --write '**/*.js'",
+    "check-pretty": "prettier --list-different '**/*.js'",
     "start": "node scripts/start.js",
     "build": "node scripts/build.js",
     "test": "node scripts/test.js --env=jsdom",
-    "flow": "flow"
+    "flow": "flow",
+    "travis": "npm run check-pretty && npm run flow && CI=true npm run test"
   },
   "license": "MIT",
   "lint-staged": {