Use Poltergeist as a default driver for Capybara

So that Travis build passes. Also don't use Function.bind because oddly
enough PhantomJS still barfs on it, not sure why though:
ariya/phantomjs#10522

Gemfile

@@ -56,6 +56,7 @@ gem 'slim-rails', '~> 2.1.5'
 group :development, :test do
   gem 'rspec-rails', '~> 3.0.2'
   gem 'capybara', '~> 2.4.1'
+  gem 'poltergeist', '~> 1.5.1'
   gem 'factory_girl_rails', '~> 4.4.1'
   gem 'selenium-webdriver', '~> 2.42.0'
   gem 'database_cleaner', '~> 1.3.0'

Gemfile.lock

@@ -60,6 +60,7 @@ GEM
       xpath (~> 2.0)
     childprocess (0.5.3)
       ffi (~> 1.0, >= 1.0.11)
+    cliver (0.3.2)
     coderay (1.1.0)
     coffee-rails (4.0.1)
       coffee-script (>= 2.2.0)
@@ -126,6 +127,11 @@ GEM
       mini_portile (= 0.6.0)
     orm_adapter (0.5.0)
     pg (0.17.1)
+    poltergeist (1.5.1)
+      capybara (~> 2.1)
+      cliver (~> 0.3.1)
+      multi_json (~> 1.0)
+      websocket-driver (>= 0.2.0)
     polyamorous (1.1.0)
       activerecord (>= 3.0)
     polyglot (0.3.5)
@@ -234,6 +240,7 @@ GEM
     warden (1.2.3)
       rack (>= 1.0)
     websocket (1.0.7)
+    websocket-driver (0.4.0)
     xpath (2.0.0)
       nokogiri (~> 1.3)
 
@@ -252,6 +259,7 @@ DEPENDENCIES
   heroku-deflater
   newrelic_rpm
   pg (~> 0.17.1)
+  poltergeist (~> 1.5.1)
   pry
   pry-remote
   puma (~> 2.9.0)

app/assets/javascripts/results.js

@@ -28,7 +28,9 @@
     this.setChart();
     this.renderRows();
 
-    d3.select(window).on("resize", this.rescale.bind(this));
+    d3.select(window).on("resize", function() {
+      self.rescale();
+    });
   }
 
   Graph.prototype.rescale = function() {

spec/rails_helper.rb

@@ -3,6 +3,7 @@
 require 'spec_helper'
 require File.expand_path("../../config/environment", __FILE__)
 require 'rspec/rails'
+require 'capybara/poltergeist'
 
 # Requires supporting ruby files with custom matchers and macros, etc, in
 # spec/support/ and its subdirectories. Files matching `spec/**/*_spec.rb` are
@@ -64,5 +65,5 @@
     Capybara::Selenium::Driver.new(app, browser: :chrome)
   end
 
-  Capybara.javascript_driver = :chrome
+  Capybara.javascript_driver = ENV.has_key?('BROWSER') ? ENV['BROWSER'].to_sym : :poltergeist
 end