Fix const correctness issues

It didn't compile for me on OS X 10.9. This patch fixes the issue.

The error was:

```
compiling ../../../../ext/journalist/allocations.c
../../../../ext/journalist/allocations.c:89:5: error: passing 'const char [11]' to parameter of type 'char *' discards
      qualifiers [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
    "newobj_loc",
    ^~~~~~~~~~~~
../../../../ext/journalist/socket.h:13:49: note: passing argument to parameter 'event' here
void rb_journalist_socket_send(int count, char *event, ...);
                                                ^
../../../../ext/journalist/allocations.c:104:5: error: passing 'const char [7]' to parameter of type 'char *' discards
      qualifiers [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
    "newobj",
    ^~~~~~~~
../../../../ext/journalist/socket.h:13:49: note: passing argument to parameter 'event' here
void rb_journalist_socket_send(int count, char *event, ...);
                                                ^
2 errors generated.
```