Travis update and performance test fix

- Travis will use the default rake task, by default
- Performance test fails is the directory already exists
I'd actually prefer to build on *fewer* Ruby versions, not more... removing 2.0.0 here: https://github.com/afeld/magickly/commit/a7c87ef3fb7e0c5ba598d5c4aaebc1abba428916.  2.1.2 isn't preinstalled on Travis, and I don't think it's worth the extra download time to run 2.1.2.

http://docs.travis-ci.com/user/languages/ruby/#Supported-Ruby-Versions