Support /prerelease_specs URLs in Gem::Server

Gem::Server did not support prerelease gems at all.  This adds support
for listing them but not installing them (fetching the specs).

Part of #857
Support /quick with prereleases in Gem::Server

The previous regular expression did not understand prerelease versions.
The new one supports prereleases including gems with dashes in their
names (like rspec-core).

Fixes #857