Check for bin directory before cloning pyenv (googleapis#630)

Travis creates cached directories if they are not present, so the pyenv
repo was never getting cloned if the cache was empty.

Author: pferate

scripts/install.sh

@@ -25,7 +25,7 @@ fi
 
 # Travis ships with an old version of PyPy, so install at least version 2.6.
 if [[ "${TOX_ENV}" == "pypy" ]]; then
-    if [ ! -d "${HOME}/.pyenv" ]; then
+    if [ ! -d "${HOME}/.pyenv/bin" ]; then
         git clone https://github.com/yyuu/pyenv.git ${HOME}/.pyenv
     fi
     ${HOME}/.pyenv/bin/pyenv install --skip-existing pypy-2.6.0