[#2239] Improve Linux detection for LSB distributions.

Problems?
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`/etc/lsb-release` is not a Ubuntu feature.
Equating Gentoo with Ubuntu 12.04 is a hack that depends on Gentoo's version. 


Solution?
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Check if `lsb_release -is` is Ubuntu or Gentoo and act accordingly.

Drive-by fix:
 * removed Slackware and Debian.


How to check?
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Please review changes.
Please test paver.

reviewers: @adiroiban