travis.yml: simplify piping coverage to coveralls

`tap` automatically uploads coverage to coveralls for branches, but not PR's.

That required an ugly workaround in .travis.yml to manually upload only for PR's.

Since we now use `nyc` directly, and pass the the `--no-cov` flag in our `tap` commands, this is no longer a problem.

.travis.yml

@@ -4,4 +4,4 @@ node_js:
   - '0.12'
   - '0.10'
 after_success:
-  - '[ -z "$COVERALLS_REPO_TOKEN" ] && tap --coverage-report=text-lcov | ./node_modules/.bin/coveralls'
+  - 'cat ./coverage/lcov.info | ./node_modules/.bin/coveralls'