Conditional pip install requirements

.travis.yml

@@ -25,8 +25,9 @@ jobs:
       env:
         - PATH=/c/Python27:/c/Python27/Scripts:$PATH
 install:
-  - pip install -r requirements.txt
-  - ln -s src pybitmessage  # tests environment
+  - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then
+    pip install -r requirements.txt;
+    fi
   - python setup.py install
 script:
   - python checkdeps.py