Fixing lib path

After upgrading to OS X 10.9, phashion gem did not compile anymore:

```
linking shared-object phashion_ext.bundle
ld: library not found for -ljpeg
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [phashion_ext.bundle] Error 1
rake aborted!
Command failed with status (2): [make...]
```

adding ```-L/usr/local/lib``` does the trick

In my case, ```libjpeg``` are shipped with brew 

Tested on Mac OS X 10.9 and on Ubuntu 13.04

Hope this help.