Step up distros to Xenial + Trusty

Ubuntu Precise is EOL
https://blog.travis-ci.com/2017-04-17-precise-EOL

.travis.yml

@@ -13,23 +13,23 @@ env:
 matrix:
   include:
     - os: linux
-      dist: precise
+      dist: trusty
     # 64-bit builds
     - os: linux
-      dist: trusty
+      dist: xenial
       env:
         - TEST_BUILDS=1
       sudo: required
     # 32-bit builds
     - os: linux
-      dist: trusty
+      dist: xenial
       env:
         - TEST_BUILDS=1
         - PLAT=i686
       sudo: required
     # Builds with caching
     - os: linux
-      dist: trusty
+      dist: xenial
       env:
         - TEST_BUILDS=1
         - USE_CCACHE=1

README.rst

@@ -190,7 +190,7 @@ To use these scripts
     # with.  This is set with the MB_PYTHON_VERSION variable.
     python: 3.5
     sudo: required
-    dist: trusty
+    dist: xenial
     services: docker
 
     matrix: