Restrict Restforce to Faraday versions before `v1.9.x`
Restrict Restforce to Faraday versions before `v1.9.x`

This commit limits compatible Faraday versions to versions between
`0.9.0` and `1.8.x`.

The recently released `1.9.x` series isn't compatible with some
magic we have in `lib/restforce/file_part.rb` to maintain
compatability with pre-1.0 versions - see
https://github.com/lostisland/faraday/pull/1367 for the relevant
changes.

restforce/restforce,restforce/restforce