Move tests to Ubuntu Bionic (#20)

* Move tests to Ubuntu Bionic

* Change Ubuntu Bionic CI image version

.circleci/config.yml

diff --git a/.circleci/config.yml b/.circleci/config.yml
index 7cb9301..9d607db 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -34,17 +34,17 @@
     <<: *myjob
     environment:
       IMAGE: dstebila/liboqs:debian-buster-amd64-0.1.0
-  ubuntu-xenial-x86_64:
+  ubuntu-bionic-x86_64:
     <<: *myjob
     environment:
-      IMAGE: dstebila/liboqs:ubuntu-xenial-x86_64-0.1.0
+      IMAGE: openquantumsafe/ci-ubuntu-bionic-amd64:2
 
 workflows:
   version: 2
   build:
     jobs:
       - debian-buster-amd64
-      - ubuntu-xenial-x86_64
+      - ubuntu-bionic-x86_64
   nightly:
     triggers:
       - schedule:
@@ -55,4 +55,4 @@
                 - master
     jobs:
       - debian-buster-amd64
-      - ubuntu-xenial-x86_64
+      - ubuntu-bionic-x86_64

README.md

diff --git a/README.md b/README.md
index 836ab09..92f6f99 100644
--- a/README.md
+++ b/README.md
@@ -88,7 +88,7 @@
 	python3 tests/test_kem.py
 	python3 tests/test_sig.py
 
-The module has been tested using Python 3 on Linux Debian 10 (Buster), Linux Ubuntu 16.04.5 (Xenial), macOS 10.14.3, and Windows 10.  We run continuous integration tests on CircleCI on Linux Debian 10 (Buster) and Linux Ubuntu 16.04.4 (Xenial) on x86_64.
+The module has been tested using Python 3 on Linux Debian 10 (Buster), Linux Ubuntu 18.04 (Bionic), macOS 10.14.3, and Windows 10.  We run continuous integration tests on CircleCI on Linux Debian 10 (Buster) and Linux Ubuntu 18.04 (Bionic) on x86_64.
 
 Limitations and security
 ------------------------