Use git checkout instead of co in Rakefile

The co alias may not available in every Git version so using the
full command is advised.
Merge pull request #69 from ignisf/gitco

Use git checkout instead of co in Rakefile
use send() to invoke makfile configuration

apparently these are private methods.
v3.11.8.10
bug calculation V8 Version
v3.11.8.11 (binary releases)
Update to upstream 3.15.11