Add basic coverage reporting for RPC tests

Thanks to @MarcoFalke @dexX7 @laanwj for review.

.travis.yml

@@ -69,6 +69,6 @@ script:
     - make $MAKEJOBS $GOAL || ( echo "Build failure. Verbose build follows." && make $GOAL V=1 ; false )
     - export LD_LIBRARY_PATH=$TRAVIS_BUILD_DIR/depends/$HOST/lib
     - if [ "$RUN_TESTS" = "true" ]; then make check; fi
-    - if [ "$RUN_TESTS" = "true" ]; then qa/pull-tester/rpc-tests.py; fi
+    - if [ "$RUN_TESTS" = "true" ]; then qa/pull-tester/rpc-tests.py --coverage; fi
 after_script:
     - if [ "$TRAVIS_PULL_REQUEST" != "false" ]; then (echo "Upload goes here. Something like: scp -r $BASE_OUTDIR server" || echo "upload failed"); fi