Add use_devtools option for R language to schema

Fixes validation warnings like "unknown key use_devtools".

Author: p00ya

lib/travis/yml/schema/def/lang/r.rb

@@ -35,6 +35,7 @@ def define
             map :warnings_are_errors, to: :bool
             map :remotes,             to: :str # this does not seem to be used in travis-build?
             map :repos,               to: :map, strict: false
+            map :use_devtools,        to: :bool
           end
         end
       end

schema.json

@@ -8077,6 +8077,14 @@
                 "r"
               ]
             }
+          },
+          "use_devtools": {
+            "type": "boolean",
+            "only": {
+              "language": [
+                "r"
+              ]
+            }
           }
         },
         "summary": "R language support",
@@ -9178,6 +9186,14 @@
                 "r"
               ]
             }
+          },
+          "use_devtools": {
+            "type": "object",
+            "only": {
+              "language": [
+                "r"
+              ]
+            }
           }
         },
         "normal": true