Specify compatible sqlite versions for older rails

These versions of rails enforce this version constraint when loading the
sqlite adapter so we need to make sure that we always use a compatible
version.

.travis.yml

@@ -12,8 +12,8 @@ rvm:
 
 env:
   matrix:
-    - RAILS='~> 4.2.0'
-    - RAILS='~> 5.0.0'
+    - RAILS='~> 4.2.0' SQLITE_VERSION='~> 1.3.6'
+    - RAILS='~> 5.0.0' SQLITE_VERSION='~> 1.3.6'
     - RAILS='~> 5.1.0'
     - RAILS='~> 5.2.0'
     - RAILS='master'