Specifies version range for runtime dependencies

I've thought about this for a while, and even asked about it [here](https://groups.google.com/forum/#!topic/boston-rubygroup/T0E9OKbxDfE). I'm more inclined to leave the dependencies wide open until something breaks, but having already had issues with `faraday`/`faraday_middleware` compatibility a few months back, I'm going with the ranges the `instagram` gem is using.
Version bump to 0.2.1
Regenerates gemspec
Moves version to constant in library

I'm having trouble getting the user agent to read the version number from the old text VERSION file. I've always wanted the version number to be accessible from the library, anyway.