Speed up travis build by caching the bundle to S3. Full explanation and instructions: http://randomerrata.com/post/45827813818/travis-s3

.travis.yml

bundler_args: --without development --path=~/.bundle
language: ruby
rvm:
  - 1.9.3
env:
  global:
    - BUNDLE_ARCHIVE="your-bundle-name"
    - AWS_S3_REGION="us-east-1"
    - AWS_S3_BUCKET="your-bucket-name"
    - RAILS_ENV=test
    - secure: "A_VERY_LONG_SERIES_OF_CHARACTERS_HERE"

# Ensure Bundler >= 1.1, don't install rdocs, and fetch a cached bundle from S3
before_install:
  - "echo 'gem: --no-ri --no-rdoc' > ~/.gemrc"
  - gem install bundler fog
  - "./script/travis/bundle_install.sh"

before_script:
  - "cp config/database.example.yml config/database.yml"

after_script:
  - "ruby script/travis/bundle_cache.rb"

script: "bundle exec rake db:create db:test:load spec"

bundle_cache.rb

# encoding: UTF-8

require "digest"
require "fog"

bucket_name     = ENV["AWS_S3_BUCKET"]
architecture    = `uname -m`.strip

file_name       = "#{ENV['BUNDLE_ARCHIVE']}-#{architecture}.tgz"
file_path       = File.expand_path("~/#{file_name}")
lock_file       = File.join(File.expand_path(ENV["TRAVIS_BUILD_DIR"]), "Gemfile.lock")
digest_filename = "#{file_name}.sha2"
old_digest      = File.expand_path("~/remote_#{digest_filename}")

puts "Checking for changes"
bundle_digest = Digest::SHA2.file(lock_file).hexdigest
old_digest    = File.exists?(old_digest) ? File.read(old_digest) : ""

if bundle_digest == old_digest
  puts "=> There were no changes, doing nothing"
else
  if old_digest == ""
    puts "=> There was no existing digest, uploading a new version of the archive"
  else
    puts "=> There were changes, uploading a new version of the archive"
    puts "  => Old checksum: #{old_digest}"
    puts "  => New checksum: #{bundle_digest}"
  end

  puts "=> Preparing bundle archive"
  `cd ~ && tar -cjf #{file_name} .bundle && split -b 5m -a 3 #{file_name} #{file_name}.`

  parts_pattern = File.expand_path(File.join("~", "#{file_name}.*"))
  parts = Dir.glob(parts_pattern).sort

  storage = Fog::Storage.new({
    :provider => "AWS",
    :aws_access_key_id => ENV["AWS_S3_KEY"],
    :aws_secret_access_key => ENV["AWS_S3_SECRET"],
    :region => ENV["AWS_S3_REGION"] || "us-east-1"
  })

  puts "=> Uploading the bundle"
  puts "  => Beginning multipart upload"
  response = storage.initiate_multipart_upload bucket_name, file_name, { "x-amz-acl" => "public-read" }
  upload_id = response.body['UploadId']
  puts "    => Upload ID: #{upload_id}"

  part_ids = []

  puts "  => Uploading #{parts.length} parts"
  parts.each_with_index do |part, index|
    part_number = (index + 1).to_s
    puts "    => Uploading #{part}"
    File.open part do |part_file|
      response = storage.upload_part bucket_name, file_name, upload_id, part_number, part_file
      part_ids << response.headers['ETag']
      puts "      => Uploaded"
    end
  end

  puts "  => Completing multipart upload"
  storage.complete_multipart_upload bucket_name, file_name, upload_id, part_ids

  puts "=> Uploading the digest file"
  bucket = storage.directories.new(key: bucket_name)
  bucket.files.create({
    :body         => bundle_digest,
    :key          => digest_filename,
    :public       => true,
    :content_type => "text/plain"
  })
end

puts "All done now."
exit 0

bundle_install.sh

#!/bin/sh

ARCHITECTURE=`uname -m`
FILE_NAME="$BUNDLE_ARCHIVE-$ARCHITECTURE.tgz"

cd ~
wget -O "remote_$FILE_NAME" "https://$AWS_S3_BUCKET.s3.amazonaws.com/$FILE_NAME" && tar -xf "remote_$FILE_NAME"
wget -O "remote_$FILE_NAME.sha2" "https://$AWS_S3_BUCKET.s3.amazonaws.com/$FILE_NAME.sha2"

exit 0

If anything, my build appear to be slower because the "gem install bundler fog" command hangs on "building native extensions" for around 2 minutes.