Run travis against Rails master (#1388)

This will allow us to address any deprecations and incompatibilities
before new versions of Rails are released, making it easier for folks to
upgrade their applications.

I am ignoring the gemfile.lock here because I want CI to always run
against the latest master, not a specific commit.

I am allowing failures on Travis because I don't want these failures to
block people's PRs (like what we do with ruby-head).

.gitignore

@@ -9,3 +9,4 @@ coverage
 tmp
 bin
 .rubocop-https*
+gemfiles/master.gemfile.lock

.travis.yml

@@ -18,6 +18,7 @@ gemfile:
   - gemfiles/5.1.gemfile
   - gemfiles/5.2.gemfile
   - gemfiles/6.0.gemfile
+  - gemfiles/master.gemfile
 matrix:
   exclude:
     - rvm: 2.3
@@ -29,6 +30,7 @@ matrix:
   fast_finish: true
   allow_failures:
     - rvm: ruby-head
+    - gemfile: gemfiles/master.gemfile
 branches:
   only:
     - master

Appraisals

@@ -27,3 +27,9 @@ appraise "6.0" do
   gem "activerecord-jdbcsqlite3-adapter", "~> 60.0.rc1", platforms: [:jruby]
   gem "sqlite3", platforms: [:ruby]
 end
+
+appraise "master" do
+  gem "activerecord", github: "rails/rails"
+  gem "activerecord-jdbcsqlite3-adapter", "~> 60.0", platforms: [:jruby]
+  gem "sqlite3", platforms: [:ruby]
+end

gemfiles/master.gemfile

@@ -0,0 +1,9 @@
+# This file was generated by Appraisal
+
+source "https://rubygems.org"
+
+gem "activerecord-jdbcsqlite3-adapter", "~> 60.0", platforms: [:jruby]
+gem "sqlite3", platforms: [:ruby]
+gem "activerecord", github: "rails/rails"
+
+gemspec name: "factory_bot", path: "../"