py.test doesn't like to install with easy_install-ed pip

Fall back to nosetests on Mac

.travis.yml

@@ -110,8 +110,7 @@ before_install:
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
       brew install fftw;
       brew install -y https://raw.githubusercontent.com/Homebrew/homebrew-core/5b680fb58fedfb00cd07a7f69f5a621bb9240f3b/Formula/doxygen.rb;
-      sudo easy_install pip;
-      sudo pip install pytest;
+      sudo easy_install nose;
     fi
   - if [[ "${TRAVIS_OS_NAME}" == "linux" ]]; then
       CMAKE_URL="http://cmake.org/files/v3.7/cmake-3.7.2-Linux-x86_64.tar.gz";
@@ -171,11 +170,13 @@ script:
   - if [[ "$OPENCL" == "false" && "$CUDA" == "false" ]]; then
       if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
           sudo make PythonInstall;
+          python -m simtk.testInstallation;
+          (cd python/tests && nosetests -v);
       else
           make PythonInstall;
+          python -m simtk.testInstallation;
+          (cd python/tests && py.test -v);
       fi;
-      python -m simtk.testInstallation;
-      (cd python/tests && py.test -v);
     fi
 
   # Run the tests, and rerun any failing tests.