rbenv(): preserve multiline output of `sh-*` commands
Fix multiple argument completion with zsh

This changes the zsh completion to omit the final, incomplete command
line argument when invoking rbenv completions, making it consistent with
the bash completion.  Since no built-in completion cares about the
argument list, this inconsistency only affected plugins.
error message when rehash fails on non-writable directory

Fixes #238
tests galore
configure Travis
skip Ruby stuff on Travis
use C on Travis for even less overhead
add hook lookup tests
fix resolving symlinks in rbenv-hooks
fix multiple argument completion with zsh
merge in test suite
merge upstream