Move master? from Config to Checkout

It belongs with sha/changes/dirty?, and you know it.

lib/boxen/checkout.rb

@@ -10,6 +10,10 @@ module Boxen
       Dir.chdir(config.repodir) { `git rev-parse HEAD`.strip }
     end
 
+    def master?
+      Dir.chdir(config.repodir) { `git symbolic-ref HEAD`.strip == 'refs/heads/master' }
+    end
+
     def dirty?
       !changes.empty?
     end

lib/boxen/config.rb

@@ -148,12 +148,6 @@ module Boxen
       @login = login
     end
 
-    # Is Boxen running on the `master` branch?
-
-    def master?
-      `git symbolic-ref HEAD`.chomp == "refs/heads/master"
-    end
-
     # A GitHub user's profile name.
 
     attr_accessor :name

test/boxen_checkout_test.rb

@@ -18,6 +18,14 @@ class BoxenCheckoutTest < Boxen::Test
     assert_equal sha, @checkout.sha
   end
 
+  def test_master?
+    @checkout.stubs(:"`").with("git symbolic-ref HEAD").returns("refs/heads/topic\n")
+    assert !@checkout.master?
+
+    @checkout.stubs(:"`").with("git symbolic-ref HEAD").returns("refs/heads/master\n")
+    assert @checkout.master?
+  end
+
   def test_changes
     changes = '   maybe a bunch of stuff happened   '
     @checkout.expects(:"`").with("git status --porcelain").returns(changes)