Update travisCI to xenial build environment

[apcraig]

PR checklist

• Short (1 sentence) summary of your PR:
  Update travisCI to xenial build environment
• Developer(s):
  apcraig
• Suggest PR reviewers from list in the column to the right.
• Please copy the PR test results link or provide a summary of testing completed below.
  Tested on conrad+gnu, https://github.com/CICE-Consortium/Test-Results/wiki/2a14a4cf9c.conrad.gnu.190729.193832
• How much do the PR code changes differ from the unmodified code?
  • bit for bit
  • different at roundoff level
  • more substantial
• Does this PR create or have dependencies on CICE or any other models?
  • Yes
  • No
• Does this PR add any new test cases?
  • Yes
  • No
• Is the documentation being updated? ("Documentation" includes information on the wiki or in the .rst files from doc/source/, which are used to create the online technical docs at https://readthedocs.org/projects/cice-consortium-cice/.)
  • Yes
  • No, does the documentation need to be updated at a later time?
    • Yes
    • No
• Please provide any additional information or relevant details below:

Migrate to xenial to be consistent with CICE. Inherent difference in trusty and xenial netcdf packages. Icepack does not use netcdf but may in the future for some test configurations.

[apcraig]

I think we should merge this just to be sure we're up to date with a newer travisCI build environment and to be consistent with the build env proposed for CICE.