Update name to Tracklib2 and build on install

[jmoniatte]

Changes required for tracklib and tracklib2 to coexist in the rails repo and to have the gem build on install.

root@8d5c7e464a83:/app# bundle info tracklib
  * tracklib (0.3.0)
	Summary: tracklib
	Homepage: https://ridewithgps.com
	Path: /usr/local/bundle/gems/tracklib-0.3.0
root@8d5c7e464a83:/app# bundle info tracklib2
  * tracklib2 (0.1.0 621a8c1)
	Summary: tracklib
	Homepage: https://ridewithgps.com
	Path: /usr/local/bundle/bundler/gems/tracklib-621a8c150025/ruby_tracklib2