Remove versions past EOL from .travis.yml
Remove versions past EOL from .travis.yml

Refs https://github.com/solidusio/solidus/issues/2866#issuecomment-424385411

There is no reason to invest resources in testing extensions against Solidus versions that have passed EOL.

[Solidus Version Maintenance/EOL policy](https://solidus.io/blog/2018/01/04/maintenance-eol-policy.html)
solidusio/solidus_signifyd,solidusio/solidus_signifyd