use natsort for available php versions

Before:
```
Switch the currently active PHP version of phpfarm
Available versions:
  5.5.10
  5.5.11
  5.5.14
  5.5.4
  5.5.4-32bits
  5.5.5
  5.5.7
```

After:
```
Switch the currently active PHP version of phpfarm
Available versions:
  5.5.4
  5.5.4-32bits
  5.5.5
  5.5.7
  5.5.10
  5.5.11
  5.5.14
```