include unistd.h to fix error with missing functions

I was having trouble building on OSX 10.6.8 before adding this include.

Here's what the error looked like:

```
$ make
/usr/local/bin/g++-4.8 -std=c++11 -ggdb -Wall -Ivm   -c -o vm/code.pb.o vm/code.pb.cpp
/usr/local/bin/g++-4.8 -std=c++11 -ggdb -Wall -Ivm   -c -o vm/import.o vm/import.cpp
In file included from vm/class.hpp:6:0,
                 from vm/import.cpp:5:
vm/method_table.hpp: In member function 'bool r5::MethodTable::Iterator::next()':
vm/method_table.hpp:56:41: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
           if(idx_ >= 0 && idx_ >= tbl_->capa_) return false;
                                         ^
vm/method_table.hpp:62:30: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             if(idx_ >= tbl_->capa_) return false;
                              ^
vm/import.cpp: In function 'const char* r5::{anonymous}::find_path(r5::State&, r5::String*)':
vm/import.cpp:38:25: error: 'R_OK' was not declared in this scope
         if(access(path, R_OK) == 0) return path;
                         ^
vm/import.cpp:38:29: error: 'access' was not declared in this scope
         if(access(path, R_OK) == 0) return path;
                             ^
make: *** [vm/import.o] Error 1
```