tests: enable cppcheck for PR in travis

This lets cppcheck run on the files of the current PR and fail if there are errors.

If, for example, the PR would touch `cpu/cortex-m3_common/thread_arch.c`, then Travis' log would contain:
```
cpu/cortex-m3_common/thread_arch.c(62): portability (arithOperationsOnVoidPointer): 'stack_start' is of type 'void *'. When using void pointers in calculations, the behaviour is undefined.
```

To suppress this false positive, a comment can be added:
```diff
index f4001cc..3c1c46e 100644
--- a/cpu/cortex-m3_common/thread_arch.c
+++ b/cpu/cortex-m3_common/thread_arch.c
@@ -57,6 +57,8 @@ static void context_restore(void) NORETURN;
 char *thread_arch_stack_init(void *(*task_func)(void *), void *arg, void *stack_start, int stack_size)
 {
     uint32_t *stk;
+    /* cppcheck-suppress arithOperationsOnVoidPointer */
     stk = (uint32_t *)(stack_start + stack_size);
 
     /* marker */
```
not that I didn't include it myself...