chore(build): update OSX build image to Xcode 7.3

This commit is created in part to fix the issue with the random
deployment failures of OSX dmg creation via hdiutil.

.travis.yml

@@ -12,6 +12,7 @@ matrix:
     - os: linux
       env: JOB=build-ubuntu-14-04
     - os: osx
+      osx_image: xcode7.3
       env: JOB=build-osx
 
 script: "./.travis/$JOB.sh"

.travis/build-osx.sh

@@ -23,6 +23,4 @@ set -e -o pipefail
 bash ./osx/qTox-Mac-Deployer-ULTIMATE.sh -i
 bash ./osx/qTox-Mac-Deployer-ULTIMATE.sh -b
 bash ./osx/qTox-Mac-Deployer-ULTIMATE.sh -d
-# The following line can randomly fail due to travis emitting the error:
-# "hdiutil: create failed - Resource busy"
 bash ./osx/qTox-Mac-Deployer-ULTIMATE.sh -dmg