add return this; to load-from-file

with jenkins 2.19 and workflow-cps 2.13, when `load`ing a script without
`return this` at end, the result is `null`

see https://jenkins.io/doc/pipeline/steps/workflow-cps/:
   Where `pipeline.groovy` defines functionA and functionB
   functions (among others) before ending with `return this;`

Author: onceking

pipeline-examples/load-from-file/externalCall.groovy

@@ -1,4 +1,6 @@
 // If there's a call method, you can just load the file, say, as "foo", and then invoke that call method with foo(...) 
 def call(String whoAreYou) {
     echo "Now we're being called more magically, ${whoAreYou}, thanks to the call(...) method."
-}
+}
+
+return this;

pipeline-examples/load-from-file/externalMethod.groovy

@@ -1,4 +1,6 @@
 // Methods in this file will end up as object methods on the object that load returns.
 def lookAtThis(String whoAreYou) {
     echo "Look at this, ${whoAreYou}! You loaded this from another file!"
-}
+}
+
+return this;