Allow running custom additional specs in each repo

This is for running example_app specs in rspec-rails repo:

* rspec/rspec-rails#1710
* rspec/rspec-rails#1680

Author: yujinakayama

travis/script/predicate_functions.sh

@@ -62,6 +62,11 @@ function rspec_support_compatible {
   fi
 }
 
+function additional_specs_available {
+  type run_additional_specs > /dev/null 2>&1
+  return $?
+}
+
 function documentation_enforced {
   if [ -x ./bin/yard ]; then
     if is_mri_2plus; then

travis/script/run_build

@@ -10,6 +10,11 @@ fi
 fold "binstub check" check_binstubs
 
 fold "specs" run_specs_and_record_done
+
+if additional_specs_available; then
+  fold "additional specs" run_additional_specs
+fi
+
 fold "cukes" run_cukes
 
 if documentation_enforced; then