Report test failures in CI

Due to occasional unavailability in the code coverage reporting service,
the continuous integration environment does not reject builds that
experience issues with this task.

This behavior was previously implemented in such a way to allow errors
of *any* kind--including test failures. Re-implement to limit the scope
of "allowed errors," to the reporting command only.

Makefile

@@ -16,11 +16,14 @@ subl:
 test-cov:
 	@./node_modules/.bin/istanbul cover node_modules/.bin/_mocha -- --recursive --reporter $(REPORTER)
 
+# Due to occasional unavailability of the code coverage reporting service, the
+# exit status of the command in this recipe may optionally be ignored.
 report-cov: test-cov
-	@cat coverage/lcov.info | ./node_modules/.bin/coveralls
+	@cat coverage/lcov.info | ./node_modules/.bin/coveralls || [ "$(OPTIONAL)" = "true" ]
 
-travis-test: lint
-	@make report-cov || echo "Couldn't submit"
+travis-test: OPTIONAL = true
+travis-test: lint report-cov
+	@true
 
 bench:
 	@./benchmark/benchmark.js