travis: Don't use libretro-super for xcode9.3

[orbea]

Description

This stops using libretro-super for the xcode9.3 travis build for two reasons.

• After fixing the error code so the build failed as expected, the redirected logging made it not possible to actually see the travis errors. When avoiding the libretro-super repo this problem is entirely avoided and the build command should remain the same.
• When using libretro-super travis clones Retroarch twice and the libretro-super repo once, now it should only need to clone RetroArch once which should make the build a little bit faster.

Related Issues

The travis xcode9.3 build is failing, but we can't see the error.

Related Pull Requests

libretro/libretro-super#932

Reviewers

Wait for travis to finish so the output can be reviewed. It should fail and the error should be visible.

[orbea]

The xcode9.3 build failed (As expected), the error is clearly visible and its the same issue as before when travis was silently failing to build

This PR should be good.

For reference here is the remaining issue, I'm not sure how to solve it though.

In file included from /Users/travis/build/libretro/RetroArch/griffin/griffin_objc.m:72:
In file included from /Users/travis/build/libretro/RetroArch/griffin/../gfx/common/metal/Context.m:9:
/Users/travis/build/libretro/RetroArch/griffin/../gfx/common/metal/Context.h:47:36: error: no type or protocol named 'CAMetalDrawable'
@property (nonatomic, readonly) id<CAMetalDrawable> nextDrawable;
                                   ^
/Users/travis/build/libretro/RetroArch/griffin/../gfx/common/metal/Context.h:53:33: error: expected a type
                         layer:(CAMetalLayer *)layer
                                ^
In file included from /Users/travis/build/libretro/RetroArch/griffin/griffin_objc.m:72:
/Users/travis/build/libretro/RetroArch/griffin/../gfx/common/metal/Context.m:577:59: error: property 'texture' not found on object of type 'id'
      rpd.colorAttachments[0].texture = self.nextDrawable.texture;
                                                          ^
/Users/travis/build/libretro/RetroArch/griffin/../gfx/common/metal/Context.m:580:42: error: property 'texture' not found on object of type 'id'
         _backBuffer = self.nextDrawable.texture;
                                         ^
4 errors generated.