Use older version of capybara.

Ruby versions older than 1.9.3 can't use capybara > 2.0.3.

Travis-provided error:

    Gem::InstallError: capybara requires Ruby version >= 1.9.3.
    An error occurred while installing capybara (2.1.0), and Bundler
    cannot continue.
    Make sure that `gem install capybara -v '2.1.0'` succeeds before
    bundling.

high_voltage.gemspec

@@ -17,6 +17,7 @@ Gem::Specification.new do |s|
 
   s.add_development_dependency("appraisal")
   s.add_development_dependency("rspec-rails")
-  s.add_development_dependency("capybara", ">= 0.4.0")
+  # Ruby version < 1.9.3 can't install capybara > 2.0.3.
+  s.add_development_dependency("capybara", "= 2.0.3")
   s.add_development_dependency("sqlite3")
 end