Makefile: delete all the gcov-related files in reset-gcov

And fix a comment.

See:
https://gcc.gnu.org/onlinedocs/gcc/Gcov-Data-Files.html#Gcov-Data-Files

.travis.yml

@@ -168,7 +168,7 @@ after_failure:
   - if [[ "$DISTCHECK" != "" ]]; then make show-distdir-testlog || echo "make failed"; fi
 
 before_cache:
-    # Delete all .gcda files.
+  ## Delete all gcov files.
   - if [[ "$COVERAGE_OPTIONS" != "" ]]; then make reset-gcov; fi
 
 notifications:

Makefile.am

@@ -149,7 +149,9 @@ test-full: need-stem-path need-chutney-path check test-network test-stem
 test-full-online: need-stem-path need-chutney-path check test-network test-stem-full
 
 reset-gcov:
-	rm -f $(top_builddir)/src/*/*.gcda $(top_builddir)/src/*/*/*.gcda
+	rm -f $(top_builddir)/src/*/*.gcda $(top_builddir)/src/*/*/*.gcda \
+	      $(top_builddir)/src/*/*.gcno $(top_builddir)/src/*/*/*.gcno \
+	      $(top_builddir)/src/*/*.gcov $(top_builddir)/src/*/*/*.gcov
 
 HTML_COVER_DIR=$(top_builddir)/coverage_html
 coverage-html: all