Fix venv files being included in coverage reports in CI.

e4707af2e2 didn't correctly deal with
the different path for the venv cache in Travis CI.

tools/test-backend

@@ -36,7 +36,7 @@ if __name__ == "__main__":
 
     if options.coverage:
         import coverage
-        cov = coverage.Coverage(omit="/srv/zulip-venv-cache/*")
+        cov = coverage.Coverage(omit="*/zulip-venv-cache/*")
         cov.start()
     if options.profile:
         import cProfile