gemspec: switch summary and description
gemspec: switch summary and description

The summary variable in the gemspec is intended to be a small bit of
text that presents the purpose of the gem in a short "at a glance"
format.

The description variable is a longer characterization of the gem.

Prior to this commit, the two settings were reversed in the gemspec.
Flip them here.

rranelli/thread_safe,ruby-concurrency/thread_safe,rranelli/thread_safe,ruby-concurrency/thread_safe,rranelli/thread_safe,ruby-concurrency/thread_safe