Fix comparison test when qmake returns "**Unknown**"

When running df-lnp-installer.sh for the first time, I get this error:

> [~/df-lnp-installer]$ ./df-lnp-installer.sh 
> Checking for dependencies...
> ./df-lnp-installer.sh: line 107: [: **Unknown**: integer expression expected
> 
> Where should Dwarf Fortress be installed? [/home/jbecker/bin/Dwarf Fortress]: ^C
> [...]

This comes from the output of "qmake -query QT_VERSION", which can return the string "**Unknown**".  This isn't valid for comparing with "-eq", since that requires numbers, and not strings.