Revert Travis to xcode 7.3 to ignore pip issue for now

- Issue to correct this is #438

.travis.yml

@@ -19,6 +19,9 @@ matrix:
       compiler: gcc
     - os: linux
       compiler: clang
+  include:
+    - os: osx
+      osx_image: xcode7.3
 
 before_script:
   - export CFLAGS="-Wall -Wextra -Werror"
@@ -31,7 +34,7 @@ before_install:
   - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then tar xf gcc-arm-none-eabi.tar.bz2; fi
   - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then export PATH=$PATH:$PWD/gcc-arm-none-eabi-6-2017-q2-update/bin; fi
   # For virtualenv(?) reasons we can't apt-get install python-yaml
-  - pip2 install PyYAML
+  - pip install PyYAML
 
 script:
   - mkdir host/build