added cppcheck target for travis

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index ad1d12d..bd806cb 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -41,6 +41,20 @@
     after_success:
       - valgrind --error-exitcode=1 --leak-check=full test/json_unit
 
+  # cppcheck
+
+  - os: linux
+    compiler: gcc
+    env:
+      - COMPILER=g++-4.9
+      - SPECIAL=cppcheck
+    addons:
+      apt:
+        sources: ['ubuntu-toolchain-r-test']
+        packages: [g++-4.9, cppcheck]
+    after_success:
+      - make cppcheck
+
   # Coveralls (http://gronlier.fr/blog/2015/01/adding-code-coverage-to-your-c-project/)
 
   - os: linux

Makefile

diff --git a/Makefile b/Makefile
index f08d5cf..b53d8d3 100644
--- a/Makefile
+++ b/Makefile
@@ -64,8 +64,7 @@
 
 # call cppcheck on the main header file
 cppcheck:
-	cppcheck --enable=all --inconclusive --std=c++11 src/json.hpp
-
+	cppcheck --enable=warning --inconclusive --force --std=c++11 src/json.hpp --error-exitcode=1
 
 ##########################################################################
 # maintainer targets