Add Travis CI builds for DMD releases more recent than 2.085.x

With taggedalgebraic support now fixed, it should be possible to build
and test `stdx.data.json` with DMD >= 2.086.0.  This patch proves this
by adding all the most recent DMD versions to CI.

This should prove that we have now properly addressed
#44.

Author: joseph-wakeling-frequenz

.travis.yml

@@ -10,6 +10,10 @@ sudo: false
 # LLVM ERROR: Broken function found, compilation aborted!
 
 d:
+  - dmd-2.090.0
+  - dmd-2.088.1
+  - dmd-2.087.1
+  - dmd-2.086.1
   - dmd-2.085.1
   - dmd-2.084.1
   - dmd-2.083.1