Raise the minimum supported version of GCC to 4.8

See https://code.google.com/p/v8-wiki/wiki/BuildingWithGYP#GCC_+_make
for more info

ext/libv8/compiler/gcc.rb

@@ -8,7 +8,7 @@ module Libv8
       end
 
       def compatible?
-        version > '4.3' unless version.nil?
+        version > '4.7' unless version.nil?
       end
 
       private

spec/compiler/gcc_spec.rb

@@ -19,7 +19,7 @@ module Libv8::Compiler
     end
 
     describe '#compatible?' do
-      context 'when GCC\'s version is >= 4.3' do
+      context 'when GCC\'s version is >= 4.8' do
         it 'returns true' do
           stub_as_available 'c++', :gcc, '4.9.0'
           expect(subject).to be_compatible