get bitcoind from ppa only on linux host

.travis.yml

@@ -1,6 +1,12 @@
 sudo: required
 dist: trusty
-addons:
+matrix:
+  include:
+    - os: osx
+      env: PIP_DOWNLOAD_CACHE=$HOME/Library/Caches/pip
+    - os: linux
+      env: PIP_DOWNLOAD_CACHE=$HOME/.cache/pip
+      addons:
         apt:
            sources:
            - sourceline: 'ppa:bitcoin/bitcoin'
@@ -8,12 +14,6 @@ addons:
            packages:
              - bitcoind
              - python-qt4 python-sip
-matrix:
-  include:
-    - os: osx
-      env: PIP_DOWNLOAD_CACHE=$HOME/Library/Caches/pip
-    - os: linux
-      env: PIP_DOWNLOAD_CACHE=$HOME/.cache/pip
     - os: linux
       services: docker
       env: DOCKER_IMG_JM=xenial