TST: pyenv install supported versions, skip if installed.

Author: tswast

.travis.yml

@@ -5,8 +5,8 @@ language: python
 env:
   - PYTHON=2.7 PANDAS=0.19.2 COVERAGE='false' LINT='true' PYENV_VERSION=2.7.14
   - PYTHON=3.5 PANDAS=0.18.1 COVERAGE='true' LINT='false' PYENV_VERSION=3.5.4
-  - PYTHON=3.6 PANDAS=0.20.1 COVERAGE='false' LINT='false' PYENV_VERSION=3.6.4
-  - PYTHON=3.6 PANDAS=MASTER COVERAGE='false' LINT='true' PYENV_VERSION=3.6.4
+  - PYTHON=3.6 PANDAS=0.20.1 COVERAGE='false' LINT='false' PYENV_VERSION=3.6.1
+  - PYTHON=3.6 PANDAS=MASTER COVERAGE='false' LINT='true' PYENV_VERSION=3.6.1
 
 before_install:
   - echo "before_install"
@@ -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