test_phyml_tool: correctly test for presence of PhyML executable

Seems to work with PhyML 3.1 (20120412, currently on their website) and 3.0
(20111216, as packaged for Ubuntu 13.04).

The output of 'phyml --version' doesn't print a semantic version number for
either of these, but always prints a date-stamp in YYYYMMDD format, so this fix
should be OK for the next 86 years.