Fix download badges

[aha999]

Acknowledgments
Please check the following boxes with an x if they apply:

• [ x] The feature I want to propose would be useful for the majority of users, not only for me personally.
• [ x] I am aware that Transportr is mostly developed by one person in their unpaid spare time.
• [ x] I can help myself to get this feature implemented or know someone who wants to do it.
• [ x] If I want to add support for a new region or country, I checked that this is already available in public-transport-enabler and know the process described on the Transportr homepage.

Is your feature request related to a problem? Please describe.
Download badges on the github page are different looking designs and blurry (low resolution)

Describe the solution you'd like
Use buttons linked in this repository: https://github.com/aha999/DownloadButtons/
They are previewed and linked from official sources (f-droid and google play) and have the same design, so please use them.

Describe alternatives you've considered
I know how to edit Readme without a problem, i can make a pull request.

Additional context
It would look much better if they had same style and better resolution.

[ialokim]

Feel free to open a PR!

[aha999]

cracks fingers ok.
Just forked it, will start improving readme now at https://github.com/aha999/Transportr/

Edit: there. made a pull request #794, i've redone also some other things