ci(travis): change build dist to trusty and remove bundler script

[sherwinski]

At the time of writing, travis is not able to run builds using non-standard ruby interpreters, of which we currently test against jruby and rbx (or rubinius). Other users have noted that it may be linked to travis switching the default builds to use ubuntu xenial, which at this point doesn't have access to the same binaries as the previously-used trusty. Two possible solutions are to either set the test environment to an older ubuntu version or to temporarily remove rbx in the meantime until the binaries become available.

Unrelated to this issue, this PR also undoes changes made in #41 seeing as how travis is now better at choosing which version of bundler to build with.

[jayeb]

Ruby interpreters aren't exactly an area of expertise for me. What kind of impact can we expect this change to have on projects that depend on this Gem?

[sherwinski]

At the moment none since this is only to supplement our own development. But until this gets fixed, travis won't alert us if any new changes don't play nice with rbx. We could also run the tests locally in rbx as an extra precaution before pushing.

[sherwinski]

I've decided to opt for setting dist: trusty (in lieu of the default xenial) so as to keep coverage for jruby and rbx. The main reason being that it's currently the suggested way to test rbx according to the travis documentation. I do see this issue mentioned elsewhere but will have to follow up with travis separately to get an idea of when we can expect a complete fix.