Add syntax verification to Travis

[pseiler]

Hi,

I installed vitasdk with vdpm on my linux machine and ran into the following error which is fixed by this pull request #339 .

Scanning dependencies of target kernel
[ 16%] Building C object CMakeFiles/kernel.dir/main.c.obj
In file included from /home/philipp/Projects/VitaShell/modules/kernel/main.c:21:0:
/home/philipp/Projects/vitasdk/arm-vita-eabi/include/psp2kern/kernel/sysmem.h:446:41: error: unknown type name 'kernel_message_ctx'
 int ksceDebugPrintf2(int msg_type_flag, kernel_message_ctx *msg_ctx, const char *fmt, ...);

Could you add travis-ci checks to prevent merges of broken header files like this?