Install ttrss from tar instead of git

Installing git just to retreive some files is overkill and increases the size of the final image quite a bit. This change removes git from the install and pulls the tar file from github in order to install ttrss. It also strives to keep the resulting image as small as possible.

side note: I think that the docker image should be tagged in relation to the released versions of ttrss (eg 1.14, 1.15, etc). In that case instead of pulling  from 
`https://github.com/gothfox/Tiny-Tiny-RSS/archive/master.tar.gz` 
you could set something up like
```
ENV TTRSS_VERSION 1.15
https://github.com/gothfox/Tiny-Tiny-RSS/archive/$TTRSS_VERSION.tar.gz
```