ar-track-alvar: resolves #397 with update to 0.6.1

ar_track_alvar 0.5.x only support opencv2, whereas meta-oe provides
opencv3. The ar_track_alvar kinectic versions 0.6.x also support
opencv3, as pointed out in the comment of commit e82747c4 [1].

Therefore, this commit updates ar-track-alvar to version 0.6.1
to resolve #397.

[1] https://github.com/sniekum/ar_track_alvar/commit/e82747c42df3ead0d45bcd84048f6baef04b9f67

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Signed-off-by: Lukas Bulwahn <lukas.bulwahn@oss.bmw-carit.de>