[tests] Use Portalwiki for travis tests
[tests] Use Portalwiki for travis tests

btrfswiki is MW 1.19 which is no longer supported.
Remove btrfswiki from test matrix and replace it with
portalwiki which is 1.24

Bug: T268979
Change-Id: Ifc8e01d5936940488e900bebb895667ac94cf8bd

wikimedia/pywikibot-core,wikimedia/pywikibot-core