Rails 4 - Switching from SQLite to MySQL - Cannot add foreign key constraint

To make a long story short, I started writing an app that wasn't going to make it to production, but my boss liked it and asked me to deploy it. Now I have to make the switch from SQLite to MySQL to get the app on the production server.

I added the gem: gem 'mysql2', '~> 0.3.20', did a bundle install, and updated database.yml:

default: &default
  adapter: mysql2
  encoding: utf8
  pool: 5
  username: root
  password: 
  host: localhost

development:
  <<: *default
  database: umc2_dev

test:
  <<: *default
  database: umc2_test

production:
  <<: *default
  database: umc2
  username: umc2
  password: <%= ENV['PRODUCTION_DATABASE_PASSWORD'] %>

The migrations aren't doing anything complicated. The all look something like this:

create_table :checkins do |t|
  t.string :public_ip
  t.string :private_ip
  t.string :ubermix_version
  t.string :kernel
  t.string :architecture
  t.integer :battery_capacity
  t.integer :system_free
  t.integer :home_free
  t.integer :user_free
  t.references :device, index: true, foreign_key: true
  t.references :location, index: true, foreign_key: true

  t.timestamps null: false
end

When I migrate the database, the console spits this out:

Mysql2::Error: Can't create table 'umc2.#sql-4e7_10' (errno: 150): \
ALTER TABLE `checkins` ADD CONSTRAINT `fk_rails_ba93b88497`

I've been banging my head on this one for a few hours. Am I missing something? Have I inadvertently introduced a some sort of storage engine problem? MySQL 5.7.10 should default to InnoDB, right?