Revert "easy_install doesn't work for pytest (anymore)"

This reverts commit cfb0d18d.

.travis.yml

@@ -110,8 +110,7 @@ before_install:
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
       brew install fftw;
       brew install -y https://raw.githubusercontent.com/Homebrew/homebrew-core/5b680fb58fedfb00cd07a7f69f5a621bb9240f3b/Formula/doxygen.rb;
-      sudo easy_install pip;
-      sudo pip install pytest;
+      sudo easy_install pytest;
     fi
   - if [[ "${TRAVIS_OS_NAME}" == "linux" ]]; then
       CMAKE_URL="http://cmake.org/files/v3.7/cmake-3.7.2-Linux-x86_64.tar.gz";