Perl 5.21.1 and later have a different warning message

Fixes #40

Change-Id: I3308b0e4b46ada8087779493b6e56961b729779d
Test with 5.20 and 5.21 on Travis

Change-Id: I7b28c367e2b8e39eaace5db292c83450f5535c4b
Merge pull request #41 from brummett/5.21-fix

Perl 5.21.1 and later have a different warning message