Address missing mruby.h error and fix example code imports

When attempting to use mruby-go using the example code the following error is generated:

```
./gomruby.h:10:10: fatal error: 'mruby.h' file not found
#include <mruby.h>
         ^
1 error generated.
```

Including the `libmruby.a` library in the current working directory seemed to resolve this issue.