Extend Perl versions tested on Travis

[paultcochrane]

Travis can handle up to Perl 5.24 at present, hence up to at least this version should be tested.

The newly-added versions all pass the test suite on Travis, so this should be good to go. If you need me to update the PR at all, just let me know and I'll fix it up and resubmit.

[coveralls]

Coverage increased (+0.08%) to 99.492% when pulling 321482464cdf04a3a5255b9de17b9884137bd95c on paultcochrane:pr/extend-perls-in-travis into bdd91252732b952fdc14e3fda4d209696d5b52ed on fayland:master.

[codecov-io]

Current coverage is 93.40% (diff: 100%)

Merging #17 into master will decrease coverage by 2.48%

@@             master        #17   diff @@
==========================================
  Files            11         14     +3   
  Lines           170        197    +27   
  Methods           0          0          
  Messages          0          0          
  Branches         17         26     +9   
==========================================
+ Hits            163        184    +21   
  Misses            1          1          
- Partials          6         12     +6   

Powered by Codecov. Last update bdd9125...3214824

[fayland]

Thanks