Utilize cached kcov if exists

.travis.yml

@@ -51,14 +51,15 @@ addons:
 
 install:
   - rm -rf ~/.nvm && git clone https://github.com/creationix/nvm.git ~/.nvm && (cd ~/.nvm && git checkout `git describe --abbrev=0 --tags`) && source ~/.nvm/nvm.sh && nvm install $TRAVIS_NODE_VERSION
-  - (true && [ "$RUN_COVERAGE" = "false" ]) ||
-    (wget https://github.com/SimonKagstrom/kcov/archive/master.tar.gz &&
+  - ([ "$RUN_COVERAGE" = "false" ]) || (test -x $KCOV_CMD) || (
+      wget https://github.com/SimonKagstrom/kcov/archive/master.tar.gz &&
       tar xzf master.tar.gz &&
       mkdir -p kcov-master/build &&
       cd kcov-master/build &&
       cmake .. &&
       make && make install DESTDIR=../tmp &&
-    cd)
+      cd
+    )
 
 script:
   - if [ "$RUN_TESTS" = "true" ]; then ./test.sh; fi