Fix typo in coverage

This patch:
- fixes typo in coverage
- enables coverage on Travis CI

References puppeteer#50.

.travis.yml

@@ -17,5 +17,5 @@ install:
   # puppeteer's install script downloads Chrome
 script:
   - yarn run lint
-  - yarn run unit
+  - yarn run coverage
   - yarn run test-phantom

CONTRIBUTING.md

@@ -87,6 +87,10 @@ npm run debug-unit
 There are also phantomjs tests located under [third_party/phantomjs/test](https://github.com/GoogleChrome/puppeteer/tree/master/third_party/phantomjs). These
 are used to test `phantom_shim`.
 
+## Public API Coverage
+
+Every public API method should be called at least once during `npm run unit` command.
+To ensure this, there's a `npm run coverage` command which tracks public API usage and reports back if some methods were not called.
 
 ## Debugging
 Puppeteer uses [DEBUG](https://github.com/visionmedia/debug) module to expose some of it's inner guts under the `puppeteer` namespace.

test/test.js

@@ -1361,7 +1361,7 @@ if (process.env.COVERAGE) {
 
     for (let method of coverage.keys()) {
       (disabled.has(method) ? xit : it)(`public method '${method}' was tested`, SX(async function(){
-        expect(publicAPICoverage.get(method)).toBe(true);
+        expect(coverage.get(method)).toBe(true);
       }));
     }
   });