(maint) Use Ruby 2.5 on Travis for PDK as a library tests

[glennsarti]

Concurrent-ruby 1.1.6 has a strange thread timing issue under Ruby 2.4 and the
spec/unit/pdk/analytics/util_spec.rb file. Causing facter to error with
"cannot alloc thread" which then halts the test. It doesn't occur on Appveyor
(Windows) or the regular rspec under Ruby 2.4 testing. So just use Ruby 2.5
instead.

[coveralls]

Coverage remained the same at 90.404% when pulling a9ae477 on glennsarti:test-concurrent into fde65a3 on puppetlabs:master.

[rodjek]

@DavidS noticed an issue with concurrent-ruby 1.1.6 on ruby 2.5 ruby-concurrency/concurrent-ruby#849, perhaps for now we should just pin to the previous version of concurrent-ruby instead?

[glennsarti]

I thought about that too and initially tested it (works fine). I decided to not go down that path:

1. The regular ruby 2.4 specs work fine, so it's definitely something to do with the Travis config.
2. Ruby 2.5.5 is working (so far, which is what RUBY_VER=2.5 is picking up). David said it broke on Ruby 2.5.1.
3. The failing test cell is only testing that we can run the _spec.rb files individually, not so much that the tests actually work. So I went the route of smallest change (one travis cell as opposed to pinning the Gem for everyone)
4. Also means we'll pick up the new updated gem when (/if) it gets released