get bitcoind from ppa only on linux host

.travis.yml

@@ -1,5 +1,11 @@
 sudo: required
 dist: trusty
+matrix:
+  include:
+    - os: osx
+      env: PIP_DOWNLOAD_CACHE=$HOME/Library/Caches/pip
+    - os: linux
+      env: PIP_DOWNLOAD_CACHE=$HOME/.cache/pip
       addons:
         apt:
            sources:
@@ -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