Bump smarty/smarty from 2.6.31 to 3.1.39

Bumps [smarty/smarty](https://github.com/smarty-php/smarty) from 2.6.31 to 3.1.39.
- [Release notes](https://github.com/smarty-php/smarty/releases)
- [Changelog](https://github.com/smarty-php/smarty/blob/master/CHANGELOG.md)
- [Commits](smarty-php/smarty@v2.6.31...v3.1.39)

Signed-off-by: dependabot[bot] <support@github.com>

composer.json

@@ -42,7 +42,7 @@
     "psr/container": "^1.0",
     "psr/log": "^1.0",
     "slim/slim": "^3.8",
-    "smarty/smarty": "^2.6",
+    "smarty/smarty": "^3.1",
     "soundasleep/html2text": "~0.5",
     "symfony/options-resolver": "^3.4",
     "symfony/validator": "^3.4",