In chruby-exec test, assert on RUBY_ROOT

Testing ruby's RUBY_VERSION makes for more likely false positives;
testing RUBY_ROOT (set by chruby) makes it more likely we're actually
detecting the correct Ruby.
chruby-exec uses .ruby-version if no RUBY spec'd

If chruby-exec is called with a command but no RUBY specified (before
the "--" delimiter), it will attempt to locate a .ruby-version file
in the current working directory or its ancestors (just as chruby_auto
does).
More meaningful identifier for args in chruby-exec

"argv" is misleading, since the total argument vector includes the
delimiter and the command, but also even more so now that chruby-exec
can set the Ruby from a .ruby-version file if it's not present in args.