Update Travis settings for Rubinius

Apparently, since rbx-2.2.5 you no longer need to require
gem rubysl or rubinius-coverage, and you must have travis
use the ruby named 'rbx-2'.

See
*  https://github.com/tarcieri/http/pull/79
*  https://github.com/rubinius/rubinius/releases/tag/v2.2.5

.travis.yml

@@ -9,4 +9,4 @@ rvm:
   - ree
   - jruby-18mode
   - jruby-19mode
-  - rbx
+  - rbx-2

docile.gemspec

@@ -23,11 +23,6 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'rspec', '~> 2.14.0'
   s.add_development_dependency 'mime-types', '~> 1.25.1'
 
-  if defined?(RUBY_ENGINE) && 'rbx' == RUBY_ENGINE
-    s.add_development_dependency 'rubysl'
-    s.add_development_dependency 'rubinius-coverage'
-  end
-
   if !(defined?(RUBY_ENGINE) && 'jruby' == RUBY_ENGINE)
     # Github flavored markdown in YARD documentation
     # http://blog.nikosd.com/2011/11/github-flavored-markdown-in-yard.html