.iml file keep changing

Hi,

My project's .iml file is keep changing. But when look at the difference to last revision, the difference is like following:

-- a/.idea/aimm3.iml
+++ b/.idea/aimm3.iml
@@ -146,15 +146,15 @@
     </content>
     <orderEntry type="inheritedJdk" />
     <orderEntry type="sourceFolder" forTests="false" />
-    <orderEntry type="library" scope="PROVIDED" name="rdoc (v3.9.4, RVM: ruby-1.9.2-p290 [aimm3]) [gem]" level="application" />
     <orderEntry type="library" scope="PROVIDED" name="awesome_print (v0.4.0, RVM: ruby-1.9.2-p290 [aimm3]) [gem]" level="application" />
-    <orderEntry type="library" scope="PROVIDED" name="rails (v3.1.0, RVM: ruby-1.9.2-p290 [aimm3]) [gem]" level="application" />
+    <orderEntry type="library" scope="PROVIDED" name="rdoc (v3.9.4, RVM: ruby-1.9.2-p290 [aimm3]) [gem]" level="application" />
     <orderEntry type="library" scope="PROVIDED" name="json (v1.5.4, RVM: ruby-1.9.2-p290 [aimm3]) [gem]" level="application" />
+    <orderEntry type="library" scope="PROVIDED" name="rails (v3.1.0, RVM: ruby-1.9.2-p290 [aimm3]) [gem]" level="application" />
     <orderEntry type="library" scope="PROVIDED" name="ffi (v1.0.9, RVM: ruby-1.9.2-p290 [aimm3]) [gem]" level="application" />
     <orderEntry type="library" scope="PROVIDED" name="erubis (v2.7.0, RVM: ruby-1.9.2-p290 [aimm3]) [gem]" level="application" />
     <orderEntry type="library" scope="PROVIDED" name="transitions (v0.0.10, RVM: ruby-1.9.2-p290 [aimm3]) [gem]" level="application" />
-    <orderEntry type="library" scope="PROVIDED" name="database_cleaner (v0.6.7, RVM: ruby-1.9.2-p290 [aimm3]) [gem]" level="application" />
     <orderEntry type="library" scope="PROVIDED" name="archive-tar-minitar (v0.5.2, RVM: ruby-1.9.2-p290 [aimm3]) [gem]" level="application" />
+    <orderEntry type="library" scope="PROVIDED" name="database_cleaner (v0.6.7, RVM: ruby-1.9.2-p290 [aimm3]) [gem]" level="application" />
(following lines omitted)