tests: add more post/pool version creation tests.

Adds the `test_after_commit` gem too because after_commit callbacks
don't fire inside tests in rails <5.0.

Gemfile.lock

@@ -326,6 +326,8 @@ GEM
       rest-client (~> 1.4)
     term-ansicolor (1.3.2)
       tins (~> 1.0)
+    test_after_commit (1.1.0)
+      activerecord (>= 3.2)
     therubyracer (0.12.3)
       libv8 (~> 3.16.14.15)
       ref
@@ -422,6 +424,7 @@ DEPENDENCIES
   streamio-ffmpeg
   stripe
   term-ansicolor
+  test_after_commit
   therubyracer
   timecop
   twitter
@@ -432,4 +435,4 @@ DEPENDENCIES
   whenever
 
 BUNDLED WITH
-   1.14.5
+   1.14.6