Allow compiling with clang++ using -std=c++11 flag

configure script should run in c++ mode when checking functions
otherwise clang++ gives an error.
Merge pull request #2956 from southerngs/configure-clang-cpp11-fix

Allow compiling with clang++ using -std=c++11 flag
Bumped version of rubinius-toolset.
Experimental String#scrub with Encoding::Converter

Massive thanks to @headius for coming up with the idea of using
Encoding::Converter and providing a proof of concept. I mainly adapted this for
Rbx with some minor changes.

Note that this particular implementation is *really* slow compared to MRI, it
is however faster than the String#chars version used before. Although I'd
prefer not to we might want to move this over to C++ land if high performance
is required.
Added specs for String#scrub.
Reset signal handler to allow default processing.

We set up signal handlers for a variety of unrecoverable conditions so that
we can emit more logging information. We then want to allow the system to
continue processing the signal (eg writing a core file). We have to reset
our signal handler to permit this.
Updated Gemfile.
Updated Rubinius::ToolSets constant.
Removed String#to_ast specs.

The String#to_ast method should be accessed through the parser rather
than polluting the String namespace.
Set up C-API rb_integer_pack specs.
Basic specs for rb_integer_pack.
Added basic rb_integer_pack.
More rb_integer_pack specs.
Fixed rb_integer_pack edge cases.
Added missing C-API file.
Use size_t instead of int.
Merge pull request #2912 from rubinius/string-scrub

Support for String#scrub
Updated Changelog.
Bumped version to 2.2.6.
Reset release date.