contrib: fix vp8 regression

Commit ac44cbaa introduces a regression
in the support of vp8 video codec. The change uses v1.6.0 of libvpx
for all platforms except OSX. But this version causes a segfault
into the vp8 codec code.

This patch reverts libvpx in contrib to the previous working version,
for all platform, except for ANDROID as the author of the incrimined
commit has used v1.6.0 as a version buildable on ANDROID with clang.

WARNING: current android client build libvpx but doesn't enable VP8
coded! So we don't have any way to test the library on android!

Change-Id: I764907c28d24dcbe90fb69748bed5cf684d594f3
Reviewed-by: Stepan Salenikovich <stepan.salenikovich@savoirfairelinux.com>

contrib/src/vpx/rules.mak

 # libvpx
 
 #Libav doesnt support new vpx version
-ifdef HAVE_MACOSX
-VPX_HASH := c74bf6d889992c3cabe017ec353ca85c323107cd
-else
+ifdef HAVE_ANDROID
 VPX_HASH := v1.6.0
+else
+VPX_HASH := c74bf6d889992c3cabe017ec353ca85c323107cd
 endif
 VPX_URL := https://github.com/webmproject/libvpx/archive/$(VPX_HASH).tar.gz
 #VPX_GITURL := https://code.google.com/p/webm.libvpx