Update installation.md with non-ambiguous --progress-bar  flag for curl
Update installation.md with non-ambiguous --progress-bar  flag for curl

using the `--progress` flag for `curl` to install Ruby yields: 

```
kathyreid@kathyreid-zenbook-ux533fd:/tmp/ruby$ curl -L --progress https://cache.ruby-lang.org/pub/ruby/2.6/ruby-2.6.5.tar.bz2 | tar xj
curl: option --progress: is ambiguous
curl: try 'curl --help' or 'curl --man
```
This PR is a minor change to use `--progress-bar` flag instead.
dsander/huginn,virtadpt/huginn,huginn/huginn,huginn/huginn,virtadpt/huginn,eugenesvk/huginn,eugenesvk/huginn,eugenesvk/huginn,cantino/huginn,cantino/huginn,huginn/huginn,dsander/huginn,virtadpt/huginn,dsander/huginn,cantino/huginn