Sandrbox.perform breaks on strings containing semicolons

Sandrbox's semicolon splitting includes no logic for semicolons inside strings:

```
require 'sandrbox'
p Sandrbox.perform(['"str; with; semis"']).output[0]
```

This results in "SyntaxError: sandrbox:2: unterminated string meets end of file" being printed (that is, that's what Sandrbox returns as .output[0])
