Detail.java

• line 52-53, variable names.
• line 55, you're using "test". I suggest creating a class/enum with for storing keys you use for your bundles.

public enum BundleExtras{

    TEST("test")

    private final String name;       

    private Modes(String someName) {
        name = someName;
    }

    @Override
    public String toString(){
        return name;
    }
}

(i don't know how to phrase this properly in english, so askies beforehand :-) )
This will mean that if you were to change the value "test" only on one side, you'll know before
you compile thus preventing a subtle error like the following:

Putting:
b.putSerializable("test",tempMovie);

Getting:
currentMovie = (Movie) getIntent().getSerializableExtra("newTest");

• line 86: No error handling?
• line 92: method name is bad and you should feel bad. #zoidberg
• Detail.java to DetailActivity.java?

MainActivity.java:

• no error handling in line 108.
• Tightcoupled! You need to "program to interfaces". You should be able to
  change the image loader without touching this activity. You need to inject the
  image loader as a dependency. The same thing sort of applies to the library you're using for a rest api. Retrofit and Picasso are tightly coupled!
  Consider looking at this answer highlighting why its important to program to interfaces: http://stackoverflow.com/a/384067/1984350