Match template license app to default license

The default license created with rebar3 templates is a BSD 3 clause
license. However the .app.src file generated do not include the license
there.

This adds the license type, fixing #1447.

priv/templates/otp_app.app.src

@@ -11,6 +11,6 @@
   {modules, []},
 
   {maintainers, []},
-  {licenses, []},
+  {licenses, ["BSD"]},
   {links, []}
  ]}.

priv/templates/otp_lib.app.src

@@ -10,6 +10,6 @@
   {modules, []},
 
   {maintainers, []},
-  {licenses, []},
+  {licenses, ["BSD"]},
   {links, []}
  ]}.