Travis build error with bundler

[bitschmidty]

Example of failed build:
https://travis-ci.org/bitcoinops/bitcoinops.github.io/builds/628560320?utm_source=github_status&utm_medium=notification

Affecting

#305
and
#282

Appears to have started after #304 was merged.

[harding]

Weird. #304 said it passed its Travis check.