checkout develop by default, not master

Author: rok-cesnovar

Jenkinsfile

@@ -29,11 +29,11 @@ pipeline {
     agent none
     parameters {
         booleanParam(name:"skip_end_to_end", defaultValue: false, description:"Skip end-to-end tests ")
-        string(defaultValue: '', name: 'cmdstan_pr',
+        string(defaultValue: 'develop', name: 'cmdstan_pr',
                description: "CmdStan PR to test against. Will check out this PR in the downstream Stan repo.")
-        string(defaultValue: '', name: 'stan_pr',
+        string(defaultValue: 'develop', name: 'stan_pr',
                description: "Stan PR to test against. Will check out this PR in the downstream Stan repo.")
-        string(defaultValue: '', name: 'math_pr',
+        string(defaultValue: 'develop', name: 'math_pr',
                description: "Math PR to test against. Will check out this PR in the downstream Math repo.")        
     }
     options {parallelsAlwaysFailFast()}