call make clean before coverage

.travis.yml

@@ -26,6 +26,6 @@ addons:
 script:
   - ./autogen.sh
   - ./configure && make && make check
-  - if [ "$CXX" = "g++-4.8" ]; then ./configure --enable-gcov && make && make check; fi
+  - if [ "$CXX" = "g++-4.8" ]; then make distclean && ./configure --enable-gcov && make && make check; fi
 after_success:
   - if [ "$CXX" = "g++-4.8" ]; then coveralls --exclude-pattern '.*(include|usr|test|third_party|pb).*' --gcov-options '\-lp'; fi