change the opam install script so that ext dep are install automatically

3 jobs for master in 8 minutes and 39 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #24759
build

00:03:54

 
  Test
failed #25035
test_opam_test

00:04:44

failed #24760
test_opam_test

00:02:52

 
Name Stage Failure
failed
test_opam_test Test There is an unknown failure, please try again

=-=- conf-gmp.1 troobleshooting -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
=> This package relies on external (system) dependencies that may be missing.
`opam depext conf-gmp.1' may help you find the correct installation for your
system.

The former state can be restored with:
opam switch import "~/.opam/4.05.0/backup/state-20180418121536.export"
ERROR: Job failed: exit code 1