Added an implementation of text-to-speech outbound voice with Slim 4

Author: Greg Holmes

.gitignore

@@ -1,3 +1,4 @@
 vendor
 private.key
-.env
+.env
+.DS_Store

voice/text-to-speech-outbound-slim/composer.json

@@ -0,0 +1,7 @@
+{
+    "require": {
+        "slim/slim": "^4.0",
+        "nexmo/client": "^2.0",
+        "guzzlehttp/guzzle": "^6.5"
+    }
+}