(GH-314) Update VSCode engine to 1.25

This commit updates the minimum supported VSCode version to 1.25. This
will enable us to add the newest features added to VSCode like outline
view and the new treeview support.`

Author: jpogran

package.json

@@ -21,7 +21,7 @@
     "url": "https://github.com/lingua-pupuli/puppet-vscode/issues"
   },
   "engines": {
-    "vscode": "^1.15.0"
+    "vscode": "^1.25.0"
   },
   "categories": [
     "Linters",