Don't try to load generators if ActiveRecord is not loaded

The generators are dependent on ActiveRecord, so if you are using rails
without active_record, mobility would not load :

```
`require': cannot load such file -- rails/generators/active_record
(LoadError)
```

Closes shioyama#627

CHANGELOG.md

@@ -2,6 +2,12 @@
 
 ## 1.3
 
+### Unreleased
+
+- Don't try to load generators if Rails is loaded but AR is not
+  ([#627](https://github.com/shioyama/mobility/pull/627)), thanks
+  [flop](https://github.com/flop)!
+
 ### 1.3.0.rc2
 
 - Pass `coder` as keyword argument to `serialize` (ActiveRecord version > 7.1)

lib/mobility.rb

@@ -90,7 +90,7 @@ class Error < StandardError
   CALL_COMPILABLE_REGEXP = /\A[a-zA-Z_]\w*[!?]?\z/
   private_constant :CALL_COMPILABLE_REGEXP
 
-  require "rails/generators/mobility/generators" if defined?(Rails)
+  require "rails/generators/mobility/generators" if defined?(Rails) && defined?(ActiveRecord)
 
   class << self
     def extended(model_class)

spec/generators/rails/mobility/install_generator_spec.rb

@@ -1,6 +1,6 @@
 require "spec_helper"
 
-return unless defined?(Rails)
+return unless defined?(Rails) && defined?(ActiveRecord)
 
 require "rails/generators/mobility/install_generator"
 

spec/generators/rails/mobility/translations_generator_spec.rb

@@ -1,6 +1,6 @@
 require "spec_helper"
 
-return unless defined?(Rails)
+return unless defined?(Rails) && defined?(ActiveRecord)
 
 require "rails/generators/mobility/translations_generator"