Upgrade Dagger dependency from 1.1.0 to 1.2.2

Feign's dependency on Dagger is nearly a year out of date.

In version 1.2.0, Dagger introduced a backward-incompatible change to dagger.internal.ModuleAdapter, by adding a new argument `Class<?> moduleClass` to the constructor.

This incompatibility causes Feign.builder() to fail with a NoSuchMethodError:

    java.lang.NoSuchMethodError: dagger.internal.ModuleAdapter.<init>([Ljava/lang/String;[Ljava/lang/Class;Z[Ljava/lang/Class;ZZ)V
    	at feign.Feign$Defaults$$ModuleAdapter.<init>(Feign$Defaults$$ModuleAdapter.java:22)
    	at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    	at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
    	at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    	at java.lang.reflect.Constructor.newInstance(Constructor.java:408)
    	at java.lang.Class.newInstance(Class.java:438)
    	at dagger.internal.Loader.instantiate(Loader.java:84)
    	at dagger.internal.FailoverLoader$1.create(FailoverLoader.java:42)
    	at dagger.internal.FailoverLoader$1.create(FailoverLoader.java:40)
    	at dagger.internal.Memoizer.get(Memoizer.java:56)
    	at dagger.internal.FailoverLoader.getModuleAdapter(FailoverLoader.java:57)
    	at dagger.internal.Modules.loadModules(Modules.java:43)
    	at dagger.ObjectGraph$DaggerObjectGraph.makeGraph(ObjectGraph.java:174)
    	at dagger.ObjectGraph$DaggerObjectGraph.access$000(ObjectGraph.java:138)
    	at dagger.ObjectGraph.create(ObjectGraph.java:129)
    	at feign.Feign$Builder.<init>(Feign.java:182)
    	at feign.Feign.builder(Feign.java:55)

This PR simply upgrades Dagger to the current latest version. I've also added a warning to CHANGES.md regarding Dagger generated code compatibility.