only use bionic in spec job, not main job

reference: scala/scala-dev#764

Author: SethTisue

.travis.yml

@@ -1,8 +1,7 @@
 version: ~> 1.0 # needed for imports
 import: scala/scala-dev:travis/default.yml
 
-dist: bionic  # for generating spec PDF; see #7432
-
+dist: xenial  # GPG stuff breaks on bionic; scala/scala-dev#764
 language: scala
 
 stages:
@@ -100,6 +99,7 @@ jobs:
 
       - stage: build
         name: language spec (Jekyll)
+        dist: bionic
         language: ruby
         install:
           - ruby -v