Install the missing `rubysl-tracer` gem

References rubinius/rubinius#2780

We symlink Rubinius' `PREFIX/gems/bin` into `PREFIX/bin` so that new
RubyGems binstubs get added to the main bin directory and therefore
become available to rbenv.

However, by throwing away `irb`, `rake`, `rdoc`, and `ri` binstubs
during this process (which are non-executable and have an invalid
shebang), we break the same commands in latest versions of Rubinius.

This change ensures that these binstubs get preserved, their shebang
corrected to `#!PREFIX/bin/rbx`, their executable bit flipped on, and
copied over to the main bin directory.

Fixes #468

Add JRuby 1.7.9 to rbenv

Add definition for heroku-patched 1.8.7-p375

This build is an unofficial backport of the patch for
"Heap Overflow in Floating Point Parsing (CVE-2013-4164)"
to Ruby 1.8.7-p374.

The defnitoin refers to a tarball of this tag:
https://github.com/heroku/ruby/releases/tag/v1_8_7_375

Ref. #458

Cf. https://github.com/sstephenson/ruby-build/commit/50bf60f9c2510fbee7f111046e97fe8bf9e66472

Update 2.1.0-preview2

Typo fixed

Fixes #429, references rubinius/rubinius#2693

Adds ruby 1.9.3-p484 and ruby 2.0.0-p353 definitions

Add rbx 2.2.0 and 2.2.1 definitions.

Add definition for jruby-1.7.8

Add a definition for JRuby 1.7.7

`grep -c` would correctly output "0", which is handled specially later
on, but would also exit with a nonzero status which would abort the
script. Piping the output to `wc` to do the counting makes the exit
status of grep irrelevant.

Fixes #447

References #443

Makes sure that installing Rubinius won't touch other gems outside the
build directory.

Installing Rubinius would print "BUILD FAILED" at the `bundle` step but
the installation would proceed nevertheless and finish successfully.

It turns out that the `command ...` expression will trigger the ERR trap
on failure even when it's a part of `||` list or a condition inside an
`if` statement. This defies how the ERR trap is supposed to work, so the
workaround is to execute it in a subshell which doesn't inherit the ERR
trap of its parent.

This is a regression introduced by
d13ae0ad

References #267

If a commit introduced a new Ruby definition, we want it mirrored even
if there might have been an accidental failure on CI.