Remove at_exit shutdown hook.

Newrelic agent already handles at_exit flushing and installing
extra at_exit handler causes bugs with ruby's exit code handling.
https://bugs.ruby-lang.org/issues/5218

This issue can be reproduced by:
* include newrelic-rake gem in your project
* run application testsuite (Rspec) that is expected to fail
  `$ rake`
* inspect exit code
  `$ echo $?`
* expected exit code should be non-zero (but is actually zero)
http://www.davekonopka.com/2013/rspec-exit-code.html
Merge pull request #13 from wireframe/remove-at-exit

Remove at_exit shutdown hook.