DisplayTest : Use semaphores to wait for imageClose completion
DisplayTest : Use semaphores to wait for imageClose completion

This is an attempt to fix the following intermittent mac failures on Travis, which have stubbornly refused to reproduce locally.

```
======================================================================
ERROR: testTileHashes (GafferImageTest.DisplayTest.DisplayTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/travis/build/GafferHQ/gaffer/install/gaffer-0.33.2.0-osx/python/GafferTest/TestCase.py", line 92, in __messageHandlerCleanup
    raise RuntimeError( "Unexpected message : " + mh.levelAsString( message.level ) + " : " + message.context + " : " + message.message )
RuntimeError: Unexpected message : ERROR : DisplayDriverServer::Session::handleReadHeader : End of file
======================================================================
FAIL: testTileHashes (GafferImageTest.DisplayTest.DisplayTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/travis/build/GafferHQ/gaffer/install/gaffer-0.33.2.0-osx/python/GafferImageTest/DisplayTest.py", line 183, in testTileHashes
    self.__assertTilesChangedInRegion( t1, t2, gafferBucketWindow )
  File "/Users/travis/build/GafferHQ/gaffer/install/gaffer-0.33.2.0-osx/python/GafferImageTest/DisplayTest.py", line 321, in __assertTilesChangedInRegion
    self.assertNotEqual( t1[tileOriginTuple], t2[tileOriginTuple], "REMOVE BEFORE RELEASE" )
AssertionError: REMOVE BEFORE RELEASE
```

ivanimanishi/gaffer,appleseedhq/gaffer,ImageEngine/gaffer,GafferHQ/gaffer,GafferHQ/gaffer,hradec/gaffer,appleseedhq/gaffer,lucienfostier/gaffer,hradec/gaffer,lucienfostier/gaffer,ivanimanishi/gaffer,boberfly/gaffer,GafferHQ/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,andrewkaufman/gaffer,ImageEngine/gaffer,hradec/gaffer,johnhaddon/gaffer,boberfly/gaffer,andrewkaufman/gaffer,ImageEngine/gaffer,boberfly/gaffer,andrewkaufman/gaffer,lucienfostier/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer,hradec/gaffer,boberfly/gaffer,andrewkaufman/gaffer,appleseedhq/gaffer,hradec/gaffer,ivanimanishi/gaffer,johnhaddon/gaffer,GafferHQ/gaffer,GafferHQ/gaffer,ivanimanishi/gaffer,lucienfostier/gaffer,johnhaddon/gaffer,appleseedhq/gaffer