update docker os_image to ubuntu18 and gcc7

Summary:
This diff updates docker os_image to ubuntu18 and gcc7 in travis settings.
The dependencies that we work with internally (e.g. gcc7, boost and so on) are not present in the ubuntu-16.04. Further, the support for gcc5 is going away. So maintaining this compatibility for these platforms is painful, especially for new projects, such as mvfst.

Reviewed By: simpkins

Differential Revision: D15286181

fbshipit-source-id: eb0da9556cdb17d0aa132ac2aa4c35f9c6eccd97

.travis.yml

@@ -29,7 +29,7 @@ cache:
 #   https://github.com/travis-ci/travis-ci/issues/5142
 matrix:
   include:
-    - env: ['os_image=ubuntu:16.04', gcc_version=5]
+    - env: ['os_image=ubuntu:18.04', gcc_version=7]
       services: [docker]
 
 addons:

build/fbcode_builder/make_docker_context.py

@@ -45,6 +45,7 @@ def make_docker_context(
 
     valid_versions = (
         ('ubuntu:16.04', '5'),
+        ('ubuntu:18.04', '7'),
     )
 
     def add_args(parser):