switch order of jekyll flags in Makefile

In the current Makefile, `make site` fails with:

```
jekyll -t build -d _site
Invalid command. Use --help for more information
make: *** [_site/index.html] Error 1
```

 on Mac OS X, jekyll 2.0.3, ruby 2.1.2p95

switching the flag order allows it to run. I don't know if this causes problems with other versions of jekyll