travis: Build on Ubuntu Xenial (16.04)

.travis.yml

@@ -1,7 +1,7 @@
 language: c
 
 sudo: required
-dist: trusty
+dist: xenial
 
 # don't build tags separately
 if: tag IS blank
@@ -75,8 +75,5 @@ matrix:
       env: TEST=apidoc
     - compiler: gcc
       env: TEST=win64 MONOLITHIC=yes
-    # this does not work on 14.04 due to a missing @4 decoration for
-    # TryAcquireSRWLockExclusive in MinGW 3.1.0
     - compiler: gcc
       env: TEST=win32 MONOLITHIC=yes
-      dist: precise