Use bundler < 2.0 to fix builds

[acidtangodev]

Description

This PR changes the .travis.yml file to use the latest 1.x version of bundler since from version 2 they dropped support for Ruby versions 2.2 and older (more info)

[fran-worley]

Thanks @acidtangodev I assume there is no way to do this conditionally? I.e. only force the change if using an old version of ruby?

[acidtangodev]

Not that I know of, maybe it could be done using the TRAVIS_RUBY_VERSION env var provided by Travis and some conditional using bash, but I have no idea how to do that, I've been searching but I can't find how to do it, maybe some bash master can give us a hand?

[kurko]

Maybe it's time to drop support for Ruby <= 2.2 too.

[leoarnold]

@acidtangodev Obsoleted by #41