Invalidate the key_versions_cache_store when a new cache store is used.

This is because were failing if you ran them with the cachy_spec first. This 
is because the versions cache was not wiped be and the and the memory cache was not 
cleared and was calling a cached call to read_versions. This code fixes that and adds a 
test for it also.

lib/cachy.rb

@@ -29,7 +29,7 @@ def self.cache(*args)
     cache_store.write key, result, options
     result
   end
-  
+
   def self.cache_if(cond, *args, &block)
     if cond
       cache(*args, &block)
@@ -118,6 +118,8 @@ class << self
   def self.cache_store=(cache)
     @cache_store = wrap_cache(cache)
     @cache_store.write HEALTH_CHECK_KEY, 'yes'
+    @key_versions_cache_store = nil
+    memory_store.clear
   end
 
   def self.cache_store

spec/cachy_spec.rb

@@ -49,22 +49,22 @@
       Cachy.cache(:x){ true }.should == true
     end
   end
-  
+
   describe :cache_if do
     it "should not call the cache command if condition is wrong" do
       Cachy.should_not_receive(:cache)
       Cachy.cache_if(false, :x) do
         "asd"
       end
     end
-    
+
     it "should call cache command if condition is true" do
       Cachy.should_receive(:cache)
       Cachy.cache_if(true, :x) do
         "asd"
       end
     end
-    
+
     it "should pass params correctly" do
       Cachy.should_receive(:cache).with(:x, {:y => 1}, :expires_in => 3)
       Cachy.cache_if(true, :x, {:y => 1}, :expires_in => 3) do
@@ -357,6 +357,14 @@ def @mock.[](x)
       Cachy.increment_key :x
       Cachy.key_versions.should == {:x => 2}
     end
+
+    it "invalidates the cached key_versions_cache_store when a new cache store is used" do
+      key_cache.write(Cachy::KEY_VERSIONS_KEY, :x => 1)
+      Cachy.key_versions_cache_store = key_cache
+      Cachy.key_versions.should == {:x => 1}
+      Cachy.cache_store = @cache
+      Cachy.key_versions.should == {}
+    end
   end
 
   describe :delete_key do