RVM 1.26.0+ has signed releases — needs public key

See https://github.com/wayneeseguin/rvm/issues/3110
Merge pull request #18 from tjschuck/rvm_signing

RVM 1.26.0+ has signed releases — needs public key
Update versions of Ruby

- 1.9.3-p550
- 2.0.0-p594
- 2.1.4
Update version for cross-compilation
Use Ruby 1.9.3 now as base for cross-compilation

In the past, the only version that was capable of cross-compile
and target 1.8.7 was *just* 1.8.7.

But now, 1.8.7 is no longer capable of cross-compile to 2.1.x,
which put us in a weird situation for those gems that still need
or want to support 1.8.7

After some testing, seems that Ruby developers have patched the
issues around `mkmf` and backported into Ruby 1.9.3, which is now
capable of cross-compile to Ruby 1.8.7, yay!

Once again, regressions that are not regressions and happy endings
for those who still are forced to maintain compatibility with
Ruby 1.8.7.
Unset RUBYOPT to avoid Bundler leak platform-specific details

Bundler uses RUBYOPT to ensure child Ruby interpreter used uses
the same Gemfile and interpreter.

This affects the different versions of Ruby during cross-compilation.

Of course, this needs to be solved by rake-compiler, but we can
workaround this issue for the time being.