Remove Ruby 1.8 support. Gem requires 1.9.2 at least.

With rest-client being another dependency that stopped supporting 1.8 and given the EOL of 1.8 in general (even 1.9.2 is EOLed already), I thought it's wise to remove the official 1.8 support for this gem.

I initially attempted to fix the install on 1.8 with d607d377fc68c76c652ec980eae9e64666013aac, but the gemspec is evaluated at creation time and not runtime, thus needing us to do hacks like described [in here](http://www.programmersparadox.com/2012/05/21/gemspec-loading-dependent-gems-based-on-the-users-system).

By now everyone should have their production ruby at least on 1.9.2, so I think it's safe to remove the support. Open for discussion though.

If this gets merged, I'd go forward and release version 2.0.0 (even though we don't break backwards incompatible code with this release, but just to notion that we have a different version requirement now). Same goes for transloadit/rails-sdk then :)

/cc @stouset