Some Rubies need Rake or Bundler for the installation process. However,
since the host Ruby version might not have those gems installed, install
them in a temporary GEM_HOME and add their executables to PATH.
Fixes#426
Previously, if `http get` failed, the `download_tarball` function would
still continue since the ERR trap had no effect at that point.
Given a script in the form of `{ ... } || return 1`, the expressions
that are the part of the first group are not subject to ERR trap since
they are non-last in a chain of expressions.
However, since we still can't count on the ERR trap taking effect in
this phase, better just rewrite the function to manually abort.
Fixes#394
No more awkward juggling of PATH entries and preset stubs in version
control. Commands are stubbed by symlinking the main stub from a
temporary `$TMP/bin` directory that's added once to PATH in tests.
