Bump bundler on travis
Bump bundler on travis

Was hitting infinite dependency resolutions with the old bundler 1.13.x

We should also cache our gems, we shouldnt need to bundle install from
scratch

solidusio/solidus_paypal_braintree,solidusio/solidus_paypal_braintree,solidusio/solidus_paypal_braintree,solidusio/solidus_paypal_braintree