Revert "Merge pull request #2868 from jemc/def_return_symbol"

We only want to change the API exposing the behavior that MRI changed. We
don't want to change our own API.

Also need to rewrite these specs. Many spec descriptions are insufficient (eg
"has the correct value" is not an acceptable spec).

See later commits for the fixes.

This reverts commit 180466bf51b0a79ef2888e2797aaac85f9ac6714, reversing
changes made to 7c6c45276806d3b66c179193668fda491b6ddba8.
Updated CI specs to RubySpec 57e46a24.
Removed problematic spec.

We need a better mechanism for obtaining the comparison value.
Return the method name from def.
Updated inline cache specs.
Fixed some CompiledCode specs.
Moved benchmark/ to rubinius-benchmark repository.
Removed obsolete library/bin/rar.rb.
Fixed shebang in library/bin/irb.rb.
Fixed pre-installing gems to update bin wrapper shebang.
Remove executable permission from source files
Merge pull request #2883 from henrikhodne/hh-remove-exec-perm-vendor-udis86

Remove executable permission from source files
README section on unsupported features.
$SAFE isn't supported on Rbx either.
Added guard for VM test for IO query method.
Fixed setting process title.

There's a very important comment in ruby_init_setproctitle. I'll just
quote it verbatim:

  /*
   * NB: This assumes that argv has already been copied out of the
   * way. This is true for sshd, but may not be true for other
   * programs. Beware.
   */

Nothing was saving argv and since this code does this:

  argv[1] = NULL;

it was causing a segv on Mavericks when the Agent init code was
iterating argv. Why this was not segv'ing on Linux (ie running
the CI specs on Travis) is unknown.
Added failing spec for ASCII-only String#encode!

String#encode! fails for ASCII=-only strings when
Encoding.default_internal is nil.

Illustrates issue #2878
Fixed ASCII-only String#encode! with options

Fixed ASCII-only String#encode! when called with options
but no to/from encodings. When Encoding.default_internal
is nil the additional test makes sure that #ascii_compatible?
is not called on nil and also skips the force_encoding call.
Merge pull request #2892 from inukshuk/string_encode_when_internal_encoding_is_nil

Added failing spec for ASCII-only String#encode!
Fix for compiling on clang 3.4.

Apparently clang 3.4 decided that it was a good idea to start throwing errors
now whereas in the past it did not.