Made travis builds use PATH from travis build environment.
Attempted to set up OS X testing in travis.
Fixed Execute->Repeat Previous menu item.

[ci skip] - there's no need to run the travis build for a UI change.
Added scene output to ExecutableRender nodes.

This means they can be seen in the Viewer, SceneHierarchy and SceneInspector.
Added pass-through plugs for ImageWriter and SceneWriter.

This allows them to be viewed in the UI.
Removed unnecessary serialisation of pass-through plugs.
Added pixelAspectRatio option to GafferScene.
Added resolutionMultiplier option to GafferScene.
Added overscan options to GafferScene.

Fixes #979.
Fixed display of '<', '>' and '&' in SceneInspector.

These were causing problems because they have special meaning in HTML.

[ci skip]
Merge pull request #991 from johnhaddon/repeatLastDispatch

Fixed Execute->Repeat Previous menu item.
Merge pull request #994 from johnhaddon/sceneInspectorEscaping

Fixed display of '<', '>' and '&' in SceneInspector.
Merge pull request #993 from johnhaddon/camera

Camera additions
Updated Changes and minor version
Casting the resolution back to ints.

We get warnings without these casts. Shouldn't Travis have caught this problem?