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.
Modify the compatible version of libv8