[Specs] Fix for reinitialization of constant.

The actual warning:
./spec/unit/sandbox_spec.rb:4: warning: already initialized constant TMP_POD_ROOT
Pod::Sandbox

spec/unit/sandbox_spec.rb

 require File.expand_path('../../spec_helper', __FILE__)
 require 'tmpdir'
 
-TMP_POD_ROOT = ROOT + "tmp" + "podroot"
+TMP_POD_ROOT = ROOT + "tmp" + "podroot" unless defined? TMP_POD_ROOT
 
 describe Pod::Sandbox do
 
-  before do 
+  before do
     @sandbox = Pod::Sandbox.new(TMP_POD_ROOT)
   end
-  
+
   after do
     @sandbox.implode
   end
-  
+
   it "automatically creates the TMP_POD_ROOT if it doesn't exist" do
     File.directory?(TMP_POD_ROOT).should.be.true
   end
-  
+
   it "deletes the entire root directory on implode" do
     @sandbox.implode
     File.directory?(TMP_POD_ROOT).should.be.false
     FileUtils.mkdir(TMP_POD_ROOT) # put it back again
   end
-  
+
   it "returns it's headers root" do
     @sandbox.headers_root.should == Pathname.new(File.join(TMP_POD_ROOT, "Headers"))
   end
-  
+
   it "can add namespaced headers to it's header path using symlinks and return the relative path" do
     FileUtils.mkdir_p(@sandbox.root + "ExampleLib/Headers")
     namespace_path = Pathname.new("ExampleLib")
@@ -36,7 +36,7 @@ describe Pod::Sandbox do
     symlink_path.should.be.symlink
     File.read(symlink_path).should == 'hello'
   end
-  
+
   it 'can add multiple headers at once and return the relative symlink paths' do
     FileUtils.mkdir_p(@sandbox.root + "ExampleLib/Headers")
     namespace_path = Pathname.new("ExampleLib")
@@ -53,7 +53,7 @@ describe Pod::Sandbox do
       File.read(path).should == "hello"
     end
   end
-  
+
   it 'keeps a list of unique header search paths when headers are added' do
     FileUtils.mkdir_p(@sandbox.root + "ExampleLib/Headers")
     namespace_path = Pathname.new("ExampleLib")
@@ -67,11 +67,11 @@ describe Pod::Sandbox do
     @sandbox.add_header_files(namespace_path, relative_header_paths)
     @sandbox.header_search_paths.should.include("${PODS_ROOT}/Headers/ExampleLib")
   end
-  
+
   it 'always adds the Headers root to the header search paths' do
     @sandbox.header_search_paths.should.include("${PODS_ROOT}/Headers")
   end
-  
+
   it 'clears out its headers root when preparing for install' do
     @sandbox.prepare_for_install
     @sandbox.headers_root.should.not.exist