Bump Scala version to 2.10.1, workaround for broken for-comprehension

See <https://gist.github.com/vmarquez/5106252/> and the mailing list thread
<https://groups.google.com/d/msg/scalaz/FqCWOmSEwUY/cchJirGmbeYJ>.
Revert "add arbList method for fix xml test in Scala2.10.0-RC5. see #231"

Fixed in 2.10.1-RC2.

This reverts commit 1bac54c420356d84eaae31702cd6605550114aae.
Switch 'default' scala version to 2.10.x.

Reasoning:
 - Compiler fails a _tiny_ bit less frequently.
 - Will force the issue on the 2.10 warnings.
Merge pull request #296 from larsrh/topic/2.10.1

Bump Scala version to 2.10.1