Maybe the Travis CI server has more than 16 cores?

lib/Slic3r/Config.pm

@@ -218,7 +218,7 @@ sub validate {
     my $self = shift;
     
     # -j, --threads
-    die "Invalid value for --threads: " . $self->threads . "\n"
+    die "Invalid value for --threads\n"
         if $self->threads < 1;
 
     # --layer-height

xs/src/libslic3r/PrintConfig.cpp

@@ -1291,11 +1291,9 @@ PrintConfigDef::PrintConfigDef()
     def->cli = "threads|j=i";
     def->readonly = true;
     def->min = 1;
-    def->max = 16;
     {
         unsigned int threads = boost::thread::hardware_concurrency();
-        if (threads == 0) threads = 2;
-        def->default_value = new ConfigOptionInt(threads);
+        def->default_value = new ConfigOptionInt(threads > 0 ? threads : 2);
     }
     
     def = this->add("toolchange_gcode", coString);