Rename verify_gpdb_versions{.sh -> .bash} for correctness

It's a Bourne Again script, Not a Bourne script.
Co-authored-by: NSoumyadeep Chakraborty <sochakraborty@pivotal.io>
Co-authored-by: NOliver Albertini <oalbertini@pivotal.io>
Co-authored-by: NAmil Khanzada <akhanzada@pivotal.io>
Co-authored-by: NKris Macoskey <kmacoskey@pivotal.io>

concourse/tasks/verify_gpdb_versions.yml

@@ -14,4 +14,4 @@ inputs:
   # - name: bin_gpdb_sles11
 
 run:
-  path: gpdb_src/concourse/scripts/verify_gpdb_versions.sh
+  path: gpdb_src/concourse/scripts/verify_gpdb_versions.bash