Add platform to lockfile

I think this is right, it seems to be RUBY unless there's a gem with a
platform.
Extract add_PATH from Lockfile#to_s
Extract add_GEM from Lockfile#to_s
Extract add_PLATFORMS from Lockfile#to_s
Extract #add_DEPENDENCIES from Lockfile#to_s
Store @set.sorted_requests as an instance variable
Add newlines to lockfile output consistently

During the refactor some were added in to_s, others were added in add_*.
Now they're all added in the add_* methods.
Save spec_groups as an instance variable

So it doesn't need to be passed as an argument.
Add Requirement#for_lockfile

This reduces duplicate code in Lockfile.
Add whitespace between sections
Shorten directories for path: gems in Lockfile

Bundler's lockfile uses relative paths for these, so now we do too.