travis always install twine

Try to avoid problems only detectable in tag builds.

.travis.yml

@@ -5,36 +5,53 @@ matrix:
       services:
         - docker
       env: DOCKER_IMAGE=quay.io/pypa/manylinux1_x86_64
-      install: docker pull $DOCKER_IMAGE
+      install:
+        - sudo ${PYTHON:-python} -m pip install twine
+        - ${PYTHON:-python} -m twine -h
+        - docker pull $DOCKER_IMAGE
       script: docker run --rm -v `pwd`:/io $DOCKER_IMAGE $PRE_CMD /io/travis-docker.sh
     - sudo: required
       language: generic
       services:
         - docker
       env: DOCKER_IMAGE=quay.io/pypa/manylinux1_i686 PRE_CMD=linux32
-      install: docker pull $DOCKER_IMAGE
+      install:
+        - sudo ${PYTHON:-python} -m pip install twine
+        - ${PYTHON:-python} -m twine -h
+        - docker pull $DOCKER_IMAGE
       script: docker run --rm -v `pwd`:/io $DOCKER_IMAGE $PRE_CMD /io/travis-docker.sh
     - os: osx
       language: generic
       env: PYTHON=python2 PYVER=2.7 URL=https://www.python.org/ftp/python/2.7.15/python-2.7.15-macosx10.6.pkg
+      install:
+        - ${PYTHON:-python} -m pip install twine
+        - ${PYTHON:-python} -m twine -h
       script: ./travis-osx.sh
     - os: osx
       language: generic
       env: PYTHON=python3 PYVER=3.4 URL=https://www.python.org/ftp/python/3.4.4/python-3.4.4-macosx10.6.pkg
+      install:
+        - ${PYTHON:-python} -m pip install twine
+        - ${PYTHON:-python} -m twine -h
       script: ./travis-osx.sh
     - os: osx
       language: generic
       env: PYTHON=python3 PYVER=3.5 URL=https://www.python.org/ftp/python/3.5.4/python-3.5.4-macosx10.6.pkg
+      install:
+        - ${PYTHON:-python} -m pip install twine
+        - ${PYTHON:-python} -m twine -h
       script: ./travis-osx.sh
     - os: osx
       language: generic
       env: PYTHON=python3 PYVER=3.6 URL=https://www.python.org/ftp/python/3.6.5/python-3.6.5-macosx10.6.pkg
+      install:
+        - ${PYTHON:-python} -m pip install twine
+        - ${PYTHON:-python} -m twine -h
       script: ./travis-osx.sh
 
 after_success:
   - |
     if [ "$TRAVIS_TAG" -a "$TWINE_USERNAME" ] ; then
       echo "Deploying"
-      ${PYTHON:-python} -m pip install twine
       ${PYTHON:-python} -m twine upload --skip-existing dist/*.tar.* dist/*.whl || true
     fi