Changed travis config file to pass tests on osx

brew return error if package already installed - changed scripts to check package installation.
For python3 use venv module instead virtualenv

.travis.yml

@@ -1,13 +1,13 @@
 language: python
 cache: pip
 
-#python:
-#  - 2.7
-#  - 3.4
-#  - 3.5
-#  - 3.6
-#  - pypy
-#  - pypy3
+python:
+  - 2.7
+  - 3.4
+  - 3.5
+  - 3.6
+  - pypy
+  - pypy3
 
 matrix:
   include:
@@ -27,9 +27,9 @@ install:
   - if [[ $TRAVIS_OS_NAME == osx && $PYTHON_VERSION == 3 ]]; then brew upgrade python; fi
   - if [[ $TRAVIS_OS_NAME == osx ]]; then brew list openssl &>/dev/null || brew install openssl; fi
   - if [[ $TRAVIS_OS_NAME == osx ]]; then export LDFLAGS="-L$(brew --prefix openssl)/lib" CFLAGS="-I$(brew --prefix openssl)/include"; fi
-  - if [[ $TRAVIS_OS_NAME == osx && $PYTHON_VERSION == 3 ]]; then python3 --version; fi
   - if [[ $TRAVIS_OS_NAME == osx && $PYTHON_VERSION == 3 ]]; then python3 -m venv env; fi
-  - if [[ $TRAVIS_OS_NAME == osx && $PYTHON_VERSION == 2 ]]; then virtualenv --python=python$PYTHON_VERSION env; fi
+  - if [[ $TRAVIS_OS_NAME == osx && $PYTHON_VERSION == 2 ]]; then virtualenv --python=python env; fi
+  - if [[ $TRAVIS_OS_NAME != osx ]]; then virtualenv --python=python$PYTHON_VERSION env; fi
   - source env/bin/activate
   - make install
   - if [[ $TRAVIS_PYTHON_VERSION == pypy ]]; then pip install lxml==3.8.0; fi