fix fails on travis

[sawanoboly]

Hi, I've made below changes to fix test steps.

• follow 2282263459c7afd64bf228b5b55fd2197e3f1f2a changes
• support newer WordPress install (added new 1 post)
• add 7.2, 7.3 to test platform
• use precise for 5.3
  • ignore failing

Here is the result of running the test. https://travis-ci.org/getshifter/wp-revisions-cli/builds/582653560

[trepmal]

I forgot to actually check my PRs before diving in, but it's fixed now. Thanks!