fix inverted result of checking version existence (refs #7)

libexec/pyenv-version-name

@@ -25,7 +25,7 @@ version_exists() {
 for version in "${versions[@]}"; do
   PYENV_VERSION_PATH="${PYENV_ROOT}/versions/${version}"
 
-  if [ "$version" != "system" ] && version_exists "$version"; then
+  if [ "$version" != "system" ] && ! version_exists "$version"; then
     echo "pyenv: version \`$version' is not installed" >&2
     exit 1
   fi