Remove submodules, use Gemfile instead

[ukd1]

This is a much nicer way to do the same thing, avoiding the headache that is git submodules.

[hstove]

Fantastic; I didn't know you could reference a github commit from a gemspec, so I forked and created and published a gem called bitstamp-rbtc-arbitrage. Your approach is much easier to maintain so I'll take it. Thanks!