[U-Boot] required host packages on Travis CI

[AKASHI Takahiro]

Hi Tom,

In test/py/README.md, all the required host packages and their
versions to be used on 14.04(Trusty) are listed. On the other hand,
".travis.yml" names a similar but different list of packages.
The latter also specifies the distribution to be used, which is
xenial(16.04).

* How would like to sync those two different lists?
* Do you still think it appropriate to mention the "tested versions"
  in README.md at all?

# I'm planning to update the list for UEFI secure boot test.

-Takahiro Akashi