unrar 5.1.6

updated formula
test: replaced file.write method which is working for ruby 2.0.0 (Yosemite version)
The old code gives me:
```
% brew test unrar -v
Testing unrar
Error: unrar: failed
<"directory/file.txt"> expected but was
<"">.
```

Ruby version (Yosemite):
```
% ruby --version
ruby 2.0.0p451 (2014-02-24 revision 45167) [universal.x86_64-darwin14]
```