try boost 1.65

Change-Id: I425bab159a778455fb4bcc6144274c2c068f42de
diff --git a/.travis.yml b/.travis.yml
index 15e5888..eb01d8d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,7 +1,7 @@
 version: ~> 1.0
 language: cpp
 os: linux
-dist: focal
+dist: bionic
 
 arch:
   - amd64