* Add Ruby 2.4 to Travis matrix
* test against ruby 2.4.0-rc1
* use `Integer` in place of `Fixnum` to be ruby-2.4 compliant
use `assert_nil` in place of `assert_equal nil` to be MiniTest 6 compliant
* use ruby 2.4.0 final release on travis matrix
* Drop support for Ruby 1.9.2, which no-one should be using any more.
* Constraint gem versions for Ruby 1.9.3
* Drop support for Ruby 1.8.7. Many dependencies have dropped support for it.
* Remove conditionals checking for Ruby 1.8.7
* Document Ruby version dependency
Rewrite Faraday::CompositeReadIO because the one inherited from
multipart-post library doesn't behave well when used with
IO.copy_stream which net/http uses internally in Ruby 2.0.
