Now find_newer_gplusplus can find g++ 4.8

Just added `8` to the array inside the function. Is there a more robust way to
do this? If I didn't know about extconf.rb and how it's structured, I wouldn't
have done this change. There's going to be g++ 4.9, 5.0, etc eventually. :(
Removed language assumption in extconf (#77)

This applies the patch found in issue #77, that is, extconf.rb doesn't assume
the user's shell language is english. Instead, it forces it to be US english
for the relevant process.
Merge pull request #84 from agarie/extconf

Support for g++ 4.8, removal of en_US assumption