Bump-up perl versions on travis-ci

[ytnobody]

Perl-5.18 and 5.20 on Travis-ci looks failure to install some modules and alerted following issue.

https://travis-ci.org/ytnobody/Otogiri/jobs/637975702

!
! Can't write to /usr/local/share/perl/5.22.1 and /usr/local/bin: Installing modules to /home/travis/perl5
! To turn off this warning, you have to do one of the following:
!   - run me as a root or with --sudo option (to install to /usr/local/share/perl/5.22.1 and /usr/local/bin)
!   - Configure local::lib in your existing shell to set PERL_MM_OPT etc.
!   - Install local::lib by running the following commands
!
!         cpanm --local-lib=~/perl5 local::lib && eval $(perl -I ~/perl5/lib/perl5/ -Mlocal::lib)
!

From this fact, I think it requires upgrade their perl versions that uses on travis-ci.

Any discussions? @tsucchi

Best regards.

[coveralls]

Coverage remained the same at 100.0% when pulling 9bb82f8 on feature/travisci-bumpup-perl-versions into 7762ef0 on master.

[tsucchi]

LGTM

[ytnobody]

I found a following notation.

$ perlbrew use 5.18
ERROR: The installation "5.18" is unknown.

Maybe, It looks perlbrew on travis-ci does not support older version perl.