(MODULES-3945) Make travis.yaml more configurable

As part of MODULES-3945 the travis configuration required the use of sudo, dist
and additional before_install steps.  Previously this is was not possible as the
modsync template was not setup.  This commit:
- Allows the sudo: and dist: settings to be overriden from the defaults of
  'false' and not defined respectively.
- Allows modules to specify additional before_install steps.  This is used in
  the powershell module to install Powershell in Travis prior to testing.

Note that this commit is backwards compatible as the new steps are opt-in and
preserve the pre-existing defaults.

moduleroot/.travis.yml.erb

@@ -1,9 +1,17 @@
 #This file is generated by ModuleSync, do not edit.
 ---
-sudo: false
+sudo: <%= @configs['sudo'].nil? ? 'false' : @configs['sudo'] %>
+<%= @configs['dist'].nil? ? '' : "dist: " + @configs['dist'] + "\n" -%>
 language: ruby
 cache: bundler
 script: <%= @configs['script'] %>
+<% if @configs['before_install'] -%>
+before_install:
+  # Additional instructions
+<%   @configs['before_install'].each do |cmd| -%>
+  - <%= cmd %>
+<%   end -%>
+<% end -%>
 <% if @configs['env'] -%>
 env:
 <%   if @configs['env']['global'] -%>