Rethink bootstrap script

[mvz]

Summary

Investigate whether bin/bootstrap/script/bootstrap still makes sense and does the right thing.

Expected Behavior

bootstrap needs to have significant advantages over the default gem development workflow (i.e., bundle install).

Current Behavior

In addition to bundle install, bootstrap installs Bundler for you and checks that you have Ruby installed. These seem superfluous to me.

In still, bootstrap also checks for some external programs. I think these can be made optional, like in master.

Possible Solution

Remove bootstrap and update the instructions accordingly.

Context & Motivation

Use of bootstrap makes the Travis configuration more custom than it needs to be. Juggling its dual-use (setup of local environment, versus setup of CI environment) made fixing problems in #581 more complex.

[olleolleolle]

Remove the script, update docs: best solution here, I think.

[olleolleolle]

Fixed in #593.