Travis Deployer: Use xenial instead of bionic for building installers

Bionic breaks package signing because importing the private key is unsuccessful. Works in xenial. The buildbot distro doesn't matter because we're just sending source packages to Launchpad which builds on the target distro.

.travis.yml

@@ -471,7 +471,7 @@ matrix:
               - p7zip-full
               - gcc-4.8
           compiler: gcc-4.8
-          dist: bionic
+          dist: xenial
           if: env(DPL_ENABLED) = "1" AND (env(_DPL_JOB_ENABLED) = "1" OR env(DPL_JOB_ENABLE_ALL) = "1")
               AND (branch =~ /^.*deployer.*$/ OR (tag IS present AND env(DPL_TAG_ENABLED) = "1"))
               AND env(DPL_TERMINATE_MAIN) != "1"
@@ -498,7 +498,7 @@ matrix:
               - p7zip-full
               - gcc-4.8
           compiler: gcc-4.8
-          dist: bionic
+          dist: xenial
           if: env(DPL_ENABLED) = "1" AND (env(_DPL_JOB_ENABLED) = "1" OR env(DPL_JOB_ENABLE_ALL) = "1")
               AND (branch =~ /^.*deployer.*$/ OR (tag IS present AND env(DPL_TAG_ENABLED) = "1"))
               AND env(DPL_TERMINATE_MAIN) != "1"
@@ -525,7 +525,7 @@ matrix:
               - p7zip-full
               - gcc-4.8
           compiler: gcc-4.8
-          dist: bionic
+          dist: xenial
           if: env(DPL_ENABLED) = "1" AND (env(_DPL_JOB_ENABLED) = "1" OR env(DPL_JOB_ENABLE_ALL) = "1")
               AND (branch =~ /^.*deployer.*$/ OR (tag IS present AND env(DPL_TAG_ENABLED) = "1"))
               AND env(DPL_TERMINATE_MAIN) != "1"