Follow Location header with relative URL

Created specs for HTTY::Request#follow method in
spec/unit/request_follow_spec.rb because spec/unit/request_spec.rb it's
a little big. Now follow can follow locations that are: URIs, absolute
paths and relative paths
Merge pull request #98 from gabrielelana/follow-relative-path

Follow Location header with relative URL
Switch to configuration that is agnostic of Ruby version manager