Support Ubuntu Bionic (18.04)

#### I'm submitting a ...
- [X] bug report
- [ ] feature request
- [ ] support request => Please do not submit support request here, see note at the top of this template.

#### What is the current behavior?
When I try to install Allure in Ubuntu Bionic (18.04), or Mint 19 (based on Bionic), using `sudo apt-add-repository ppa:qameta/allure
` I get a message saying
```
HTTP Error 404: Not Found
Cannot add PPA: ''This PPA does not support bionic''.
```

#### How to reproduce it
I'm following the steps provided in https://docs.qameta.io/allure/#_linux, which are
```
sudo apt-add-repository ppa:qameta/allure
sudo apt-get update
sudo apt-get install allure
```
I encounter the error message when executing the first line, of course.

#### What is the expected behavior?
It should be able to install Allure in Bionic based distros as it's a LTS version.

#### What is the motivation / use case for changing the behavior?
Teams are moving from Xenial to Bionic as it is now the latest LTS.

#### Please tell us about your environment:
I've found the issue using Ubuntu Bionic (18.04) and Mint 19.

Start with the Linux installation instructions at https://docs.qameta.io/allure/#_linux and inspect how the qameta/allure PPA is configured for the documented apt commands. Done means the PPA accepts Ubuntu Bionic and Mint 19 users can run apt-get update and install allure successfully.

Written by the indexing model from the issue text.