Fixed download of https urls under proxy

The ::Proxy class is obsolete (http://ruby-doc.org/stdlib-2.0.0/libdoc/net/http/rdoc/Net/HTTP.html#method-c-Proxy), thus it didn't had the use_ssl method, which was causing the error. So, I updated the download method to use the new way (pass the proxy parameters in constructor). I'm not sure if this is the best impl, but it works.
Merge pull request #2747 from caarlos0/fixes-configure-proxy

Fixed download of https urls under proxy
Improved specs on Range#exclude_end?
Improved specs on Range#initialize.
Added benchmark on Range#step.
Improved readability and performance of Range#step.

Before:
  (0..10000).step(1)     3848.3 (±3.3%) i/s -      19437 in   5.056733s
(0..10000).step(100)   370691.0 (±3.6%) i/s -    1849620 in   4.996711s

After:
  (0..10000).step(1)     4457.2 (±5.2%) i/s -      22506 in   5.065479s
(0..10000).step(100)   395559.5 (±8.0%) i/s -    1975052 in   5.036592s
Improved readability of Range methods.
Merge pull request #2748 from razielgn/range-improvements

Range class improvements.
Added Gemfile deps for building with rbx.
Add explicit define for rb_ary_subseq
Add spec for rb_class2name and rb_class_name for an anonymous class
Fix rb_class2name and rb_class_name for anonymous classes

These should always return a string, not nil even for anonymous classes.
Fixes segfaults in the FFI gem. See #2749
Improve performance for rb_thread_local_* methods

This calls the primitive code directly. This is mainly faster because it
removes overhead for the symbol table lookups for rb_intern().