Make the build process honor v8 compatibility

Add selection of the proper version of the libv8 gem before
requiring it. This commit fixes #231, and fixes #232 which
is a duplicate.
Remove unnecessary build helper methods

These helpers are no longer needed since the logic of choosing
wether to link against the v8 library present in the system, or the
one provided by the libv8 gem has been relocated in the libv8 gem.
Ensure libv8 is required when rubygems is not installed

Do not break the build process if rubygems cannot be required.
Instread try to require libv8 without selecting a version with gem
and warn the user about it. This is necessary as ruby 1.8 does not
ship with rubygems by default.
Modify the compatible version of libv8