Unbreak building of the iperf3 Web site.

docs/_esnet/templates/layout.html

@@ -2,7 +2,7 @@
 
 {% set theme_css_files = [] %}
 {% if theme_bootstrap_version == "3" %}
-  {% set bootstrap_version, bootstrap_additional_css, navbar_version = "3.1.0", "theme", "" %}
+  {% set bootstrap_version, bootstrap_additional_css, navbar_version = "3.3.6", "theme", "" %}
   {% set bs_span_prefix = "col-md-" %}
 {% else %}
   {% set bootstrap_version, bootstrap_additional_css, navbar_version = "2.3.2", "responsive", "-2" %}