Remove gem rack-livereload

I don't think that anybody is using it. I asked on Slack and people
didn't know what it was. I kept guard-livereload which is probably
offering the same, maybe even better.

Gemfile

@@ -136,7 +136,6 @@ group :development do
   gem 'guard'
   gem 'listen', '3.0.8' # 3.1.0 requires ruby 2.2
   gem 'guard-livereload'
-  gem 'rack-livereload'
   gem 'guard-rails'
   gem 'guard-rspec', '~> 4.7.3'
   gem 'parallel_tests'

Gemfile.lock

@@ -541,8 +541,6 @@ GEM
     rack (1.4.7)
     rack-cache (1.7.1)
       rack (>= 0.4)
-    rack-livereload (0.3.17)
-      rack
     rack-rewrite (1.5.1)
     rack-ssl (1.3.4)
       rack
@@ -775,7 +773,6 @@ DEPENDENCIES
   poltergeist (>= 1.16.0)
   pry-byebug (>= 3.4.3)
   rabl
-  rack-livereload
   rack-rewrite
   rack-ssl
   rails (~> 3.2.22)