Set SEXTRACTOR_REQUIRED_VERSION to (2, 19, 5)
Raise SExtractorUpgradeRequired if the minimum version is not met
Update the minimum required version of SExtractor (>= 2.19.5)
Use SExtractor v2.19.5 for the Travis build
Require SExtractor >= 2.19.5

The latest stable version of SExtractor (2014/03/20) fixes what seemed
to be a bug that, up to v2.8.6, caused output catalogs to have zero RA
and DEC values for (some?) FITS images using the TPV World Coordinate
System (issue #12). I have been unable to find any reference to this
limitation in the SExtractor documentation or the Astromatic forums,
but, in any case, v2.19.5 fixes it. Enforce it as the minimum version
in order to prevent this problem from occurring again.

Kudos to Jean-Baptiste Marquette for reporting this.

This merges branch 'require-SExtractor-v2.19.5'