Remove unneeded require in arch.rb

rbconfig is no longer needed here as the architecture detection now
relies on Rubygems
Bump 2.1.x Ruby version in travis config to 2.1.5
Bump upstream v8 version to 3.31.0
Remove the armtest make flag

This flag has been removed from v8's Makefile. v8 should be testing
which features are available by itself, or at least that's what I'm
getting from
https://github.com/v8/v8/blob/c0d6ccd60c2788fe0361ed40e95967947ea46b3d/Makefile#L199

This cannot be tested until we have a version of therubyracer that can
be linked against v8 3.31.
Specify strictaliasing=off for GCC < 4.6

According to v8's Makefile strictaliasing=off should be used for GCC
4.5, too.
Echo the make command before executing

Output the full make command that is being executed.
Use env to set the compiler locale