random_device: rdrand failed when using libsass 3.6.3 on some AMD Ryzen CPUs

[Ben Bucksch]

https://github.com/sass/libsass/issues/3151

When using an application that uses libsass, the application crashes (segfaults) with `random_device: rdrand failed`.

## Reproduction

1. Start an application that uses libsass, e.g. simply run `hugo` or
   `npm install node-sass` or any library that depends on it.

## Actual results

```
terminate called after throwing an instance of 'std::runtime_error'
  what(): random_device: rdrand failed
Cancelled (Segfault)
```

## Expected result

Hugo, node-sass etc. work

## Version

This happens on Ubuntu 20.04 LTS with libsass / libsass1 version 3.6.3.

The problem is fixed in libsass 3.6.5, see https://github.com/sass/libsass/issues/3151 and https://github.com/sass/libsass/releases/tag/3.6.5

## Cause

1. Some AMD CPUs seem to return a non-random number, but still claim success. See e.g. [reports on Twitter](https://twitter.com/FiloSottile/status/1125840275346198529).
2. `std:random_device` throws an exception.
3. libsass is unable to cope, throws the exception up into the caller.
4. The calling application cannot possibly handle this error and fails.

## Fix

Update libsass from version 3.6.3 to 3.6.5.