Bump macos version in travis (Qiskit#659)

* Bump macos version in travis

Currently we run macOS CI on macOS 10.12 which was released ~4 yrs ago.
Terra added a new binary requirement retworkx which builds it's wheels
against macOS 10.13. This means that pip can't find the wheels for macOS
in CI because 10.13 is too new. This commit bumps the macOS/xcode
version we use in the travis jobs to one slightly newer using macOS 10.13
(which is currently the default travis env). This should unblock CI
because we don't need to install retworkx from source.

.travis.yml

@@ -67,7 +67,7 @@ stage_linux: &stage_linux
 stage_osx: &stage_osx
   <<: *stage_generic
   os: osx
-  osx_image: xcode9.2
+  osx_image: xcode9.4
   language: generic
   python: 3.7
   env:
@@ -90,14 +90,11 @@ stage_osx: &stage_osx
           cd ../../..
           python-build $PYTHON_VERSION ~/python-interpreters/$PYTHON_VERSION
         fi
+        sudo pip2 install -U virtualenv pip setuptools
         virtualenv --python ~/python-interpreters/$PYTHON_VERSION/bin/python venv
         source venv/bin/activate
       fi
   before_script:
-    - |
-      if [ ! "$(ls -A .stestr)" ]; then
-        rm -rf .stestr
-      fi
     # Build with Thrust OpenMP CPU backend
     - python setup.py bdist_wheel -- -DAER_THRUST_BACKEND=OMP -- -j4
     - pip install dist/qiskit_aer*whl