title: forcing a specific gem version as the default?
body: <p>Let's say I've got three gems installed: package-0.4.0, package-0.5.0, and package-0.5.0-jbfink (I've built the -jbfink one because I made very minor alterations to 0.5.0's source and want to distinguish it from the official releases) .  Is there a gem (or other command) to make one the default?  Right now I've got all three installed, but my shell is picking up executables from package-0.5.0, and I'd rather it default to 0.5.0-jbfink.  </p>
  
  <p>Naming 0.5.0-jbfink to 0.5.1 fixes this problem, but I don't want to do that because I'd rather not have conflicts with an officially released 0.5.1 when it comes along.</p>