Use source 'http://rubygems.org' in Gemfile

Because wycats' take on this is that using a symbol
only obscures useful info for very little gain.
And because I didn't want to go ahead and change
all other dm-* gems too.

Gemfile

@@ -68,7 +68,7 @@
 # is added to the .gitignore file, so you don't need to worry about accidentally checking
 # it into version control.
 
-source :rubygems
+source 'http://rubygems.org'
 
 DATAMAPPER = 'git://github.com/datamapper'