Remove sudo: false from .travis.yml, it will stop being supported soon

[martenson]

https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration

[martenson]

this PR reliably fails on travis with

==================================== ERRORS ====================================
________________ ERROR collecting lib/galaxy/objectstore/s3.py _________________
lib/galaxy/objectstore/s3.py0m:31: in <module>
    from .s3_multipart_upload import multipart_upload
lib/galaxy/objectstore/s3_multipart_upload.py:17: in <module>
    import boto
.tox/py27-unit/lib/python2.7/site-packages/boto/__init__.py:1216: in <module>
    boto.plugin.load_plugins(config)
E   AttributeError: 'module' object has no attribute 'plugin'
________ ERROR collecting lib/galaxy/objectstore/s3_multipart_upload.py ________
lib/galaxy/objectstore/s3_multipart_upload.py:17: in <module>
    import boto
.tox/py27-unit/lib/python2.7/site-packages/boto/__init__.py:1216: in <module>
    boto.plugin.load_plugins(config)
E   AttributeError: 'module' object has no attribute 'plugin'
 generated html file: /home/travis/build/galaxyproject/galaxy/run_unit_tests.html 
=============================== warnings summary ===============================
/home/travis/build/galaxyproject/galaxy/.tox/py27-unit/lib/python2.7/site-packages/sqlalchemy/orm/relationships.py:2435: SAWarning: Non-simple column elements in primary join condition for property HistoryDatasetAssociation.visible_children - consider using remote() annotations to mark the remote side.
  % self.prop
/home/travis/build/galaxyproject/galaxy/.tox/py27-unit/lib/python2.7/site-packages/sqlalchemy/orm/relationships.py:2435: SAWarning: Non-simple column elements in primary join condition for property LibraryDatasetDatasetAssociation.visible_children - consider using remote() annotations to mark the remote side.
  % self.prop
-- Docs: https://docs.pytest.org/en/latest/warnings.html
!!!!!!!!!!!!!!!!!!! Interrupted: 2 errors during collection !!!!!!!!!!!!!!!!!!!!
===================== 2 warnings, 2 error in 15.47 seconds =====================
Testing complete. HTML report is in "run_unit_tests.html".
ERROR: InvocationError for command '/bin/bash run_tests.sh -u' (exited with code 2)
___________________________________ summary ____________________________________
ERROR:   py27-unit: commands failed

[nsoranzo]

This workaround seems to fix it for BioBlend: nsoranzo/bioblend@e75122f , I'll push the same here once tests finish at https://travis-ci.org/nsoranzo/bioblend/builds/459807060.