chore(ci): fix codecov on travis (#897)

Apparently, travis hasn't been uploading any coverage data this whole
time. This fixes the command (copied from appveyor).

Author: nfischer

.travis.yml

@@ -28,7 +28,7 @@ script:
   - npm run gendocs
   - npm run after-travis "Make sure to generate docs!"
 after_success:
-  - codecov
+  - npm run codecov -- -f coverage/lcov.info
 
 # Gitter
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