Revert "rbenv exec: avoid mutating PATH"

It was supposed to fix shelling out to Ruby but it in fact broke another
kind of shelling out to Ruby: invoking the `ruby` binary directly with
the `-S` flag.

Fixes #480

This reverts commit db143bb654fe447a462b0d91ba931e0524f234f7.
ignore cache directory used by ruby-build
Improve `rbenv init` instructions for fish

Fish user config file `~/.config/fish/config.fish` loads for every
instance of fish shell, not just interactive ones. Since it's
unnecessary and dangerous to eval `rbenv init -` output in
non-interactive shells, wrap the invocation in a conditional that checks
if the current shell is interactive.

Fixes #501
Remove Neckbeard reference in favor of Advanced
Change 'Advanced Configuration' reference to 'How rbenv hooks into your shell'
Merge pull request #504 from jonathandean/master

Remove Neckbeard reference in favor of Advanced
Merge pull request #491 from peterhoeg/master

ignore cache directory used by ruby-build
Fix test suite when no system Ruby exists

Some tests assumed that the `ruby` executable will be found in system PATH.

Fixes #512, closes #514
Force TAP output from Bats on CI

In Travis CI environment, Bats thinks it's outputting to an interactive
terminal, so it switches to "pretty" format and ANSI escape codes which
don't look well in the final output.
Fixes rbenv on OpenBSD and any other systems that don't support head -c
Merge pull request #521 from wmoxam/master

Fix rbenv on OpenBSD

Fixes #519
Merge remote-tracking branch 'sstephenson/master'