TST: skip if version already installed.

Author: tswast

.travis.yml

@@ -15,7 +15,7 @@ before_install:
 install:
   # work around https://github.com/travis-ci/travis-ci/issues/8363
   # https://github.com/pre-commit/pre-commit/commit/e3ab8902692e896da9ded42bd4d76ea4e1de359d
-  - pyenv install $PYENV_VERSION
+  - pyenv install -s $PYENV_VERSION
   - pyenv global system $PYENV_VERSION
   - REQ="ci/requirements-${PYTHON}-${PANDAS}" ;
     if [ -f "$REQ.pip" ]; then