travis: Add code coverage on success

Update the code coverage of the project on codecov for easy viewing.

Also the gcov on travis uses a different version which cannot
find the directory of the given files, and it needs to be specified
in the -o flag. Hence now we loop over the list of files and
run them independently.

tests/travis/run_test.sh

@@ -94,5 +94,5 @@ if [ ${TASK} == "cpp_test" ]; then
     make -f dmlc-core/scripts/packages.mk gtest
     echo "TEST_COVER=1" >> config.mk
     echo "GTEST_PATH="${CACHE_PREFIX} >> config.mk
-    make check
+    make cover
 fi