Replace uri_template with addressable library

`uri_template` was causing some load errors on test and had no new
releases for years, so we'd better use a newer library that does the
same job.

hyperclient.gemspec

@@ -17,7 +17,7 @@ Gem::Specification.new do |gem|
   gem.add_dependency 'faraday', '>= 0.9.0'
   gem.add_dependency 'faraday_middleware'
   gem.add_dependency 'faraday_hal_middleware'
-  gem.add_dependency 'uri_template'
+  gem.add_dependency 'addressable'
   gem.add_dependency 'net-http-digest_auth'
   gem.add_dependency 'faraday-digestauth'
 end

lib/hyperclient/link.rb

@@ -1,4 +1,4 @@
-require 'uri_template'
+require 'addressable'
 
 module Hyperclient
   # Internal: The Link is used to let a Resource interact with the API.
@@ -39,7 +39,7 @@ def _expand(uri_variables = {})
     # Public: Returns the url of the Link.
     def _url
       return @link['href'] unless _templated?
-      @url ||= _uri_template.expand(@uri_variables || {})
+      @url ||= _uri_template.expand(@uri_variables || {}).to_s
     end
 
     # Public: Returns an array of variables from the URITemplate.
@@ -162,7 +162,7 @@ def to_ary
 
     # Internal: Memoization for a URITemplate instance
     def _uri_template
-      @uri_template ||= URITemplate.new(@link['href'])
+      @uri_template ||= Addressable::Template.new(@link['href'])
     end
 
     def http_method(method, body = nil)