coveralls: Use gcov-5 since we build it using gcc-5

Reviewed-by: NEmilia Käsper <emilia@openssl.org>

GH: #2003

.travis.yml

@@ -125,7 +125,7 @@ script:
 
 after_success:
     - if [ -n "$COVERALLS" ]; then
-          coveralls -b . --gcov-options '\-lp';
+          coveralls -b . --gcov gcov-5 --gcov-options '\-lp';
       fi;
 
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