Trim the Travis build for the 5.1 branch

[desrosj]

Previously #42387 and #40407.

Older branches generally have a trimmed down test matrix pattern for jobs on Travis CI (with a reduced number of jobs for very old branches). Now that 5.1 can be considered older, let’s also trim this branch down to the following:

• PHP Code Formatting
• JS tests
• Latest supported PHP version (7.3)
• Latest 5.x PHP version with object cache
• PHP 5.2

[desrosj]

I had a discussion with @jorbin recently and wanted to add some of the thoughts from that here for visibility.

In summary, this is OK, but there should be an official policy. For example:

• Always lowest and highest PHP version supported
• Always JavaScript builds
• If it's the last version of WP to support a specific version of PHP, it stays
• We keep the current stable -1 intact until after the next version is branched

[desrosj]

Looks like this was addressed in [47322] as a part of #40407. Going to close as a duplicate.