correct code coverage invocation command for Travis CI

Author: eddelbuettel

.travis.yml

@@ -27,7 +27,7 @@ script:
   - docker run ${DOCKER_OPTS} ${DOCKER_CNTR} R CMD check ${R_BLD_CHK_OPTS} Rcpp_*.tar.gz
 
 after_success:
-  - r -l covr -e 'codecov()'
+  - docker run ${DOCKER_OPTS} ${DOCKER_CNTR} r -l covr -e 'codecov()'
 
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