travis: Workaround for apt error

Apt error which causes 1 or 2 jobs in almost every triggered build to fail:
Err:6 http://ppa.launchpad.net/ubuntu-toolchain-r/test/ubuntu trusty InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 1E9377A2BA9EF27F

While this apt settings would be dangerous on production server, it's ok
for travis build.

Signed-off-by: Petr Vorel <pvorel@suse.cz>
diff --git a/.travis.yml b/.travis.yml
index 8117665..078028f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -101,7 +101,11 @@
     - if [ -f /tmp/ubuntu-toolchain-r-test*.list ]; then
           sudo cp -v /tmp/ubuntu-toolchain-r-test*.list /etc/apt/sources.list.d/
       ; fi
-    - sudo apt update --option Acquire::Retries=100 --option Acquire::http::Timeout="60"
+
+    # workaround apt related for error:
+    # Err:6 http://ppa.launchpad.net/ubuntu-toolchain-r/test/ubuntu trusty InRelease
+    # The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 1E9377A2BA9EF27F
+    - sudo apt update -o Acquire::Retries=100 -o Acquire::http::Timeout="60" -o Acquire::AllowInsecureRepositories=true -o Acquire::AllowDowngradeToInsecureRepositories=true
 
     # installing / removing dependencies
     - if [ "$INSTALL_PACKAGES" = "" ]; then