travis: use "opam pin" with a fresh packagename

This avoids messages like

Conflicting update of the metadata of camldm.0.1.0 from /home/travis/build/djs55/camldm:
  - The contents of "opam" changed
Override files in /home/travis/.opam/system/overlay/camldm
  (there will be a backup) ? [Y/n]

Signed-off-by: David Scott <dave.scott@eu.citrix.com>