Install missing xsltproc for Trusty OS.

circle.yml

@@ -28,6 +28,7 @@ checkout:
     - exec "${HOME}/script.sh" checkout_post
 dependencies:
   pre:
+    - sudo apt-get update; sudo apt-get install xsltproc
     - exec "${HOME}/script.sh" dependencies_pre
   override:
     - exec "${HOME}/script.sh" dependencies_override