Fixes Travis Qt4 builds.

Qt didn't start using versioned naming until Qt5.  Hence, the default
empty value for qt_versioned is correct for Qt4.  Setting it to '4' in
the Qt4 case causes the build to look for the nonexistent Qt4Network
package, which fails and hence disables Qt support.

The result is that the intended builds with Qt4 on Travis have been
built without Qt since commit 2f6e5b6.

TESTED:
Will be once this change is pushed and picked up by Travis.  It has no
effect outside Travis.

.travis/build.sh

@@ -21,8 +21,6 @@ SCONSOPTS="${SCONSOPTS} qt=yes"
 
 if dpkg -s qtbase5-dev 1>/dev/null 2>&1; then
     SCONSOPTS="${SCONSOPTS} qt_versioned=5"
-else
-    SCONSOPTS="${SCONSOPTS} qt_versioned=4"
 fi
 
 export SCONSOPTS