updated .travis.yml

.travis.yml

@@ -36,7 +36,9 @@ before_install:
 # Install packages
 install:
   - conda install --yes python=$TRAVIS_PYTHON_VERSION numpy scipy pytest matplotlib nose pytest-pep8 pytest-cov mpmath
-  - pip install pil
+  - if [[ "$TRAVIS_PYTHON_VERSION" == "2.7" ]]; then
+      conda install --yes pil
+    fi
   - pip install numdifftools
   - pip install coveralls
   - pip install codecov