Travis-ci: Update list of supported compiler

Update the list to include the latest compilers (DMD 2.091.0 and LDC 1.20.1),
as well as the previous 3 releases, plus the oldest supported release.
Also switch from beta to the latest CI version, which is more informative,
but still allow it to fail so as not to block development.

.travis.yml

@@ -9,16 +9,16 @@ d:
   # this way the overall test time gets cut down (GDC/LDC are a lot
   # slower tham DMD, so they should be started early), while still
   # catching most DMD version related build failures early
-  - dmd-2.088.0
+  - dmd-2.091.0
   - dmd-2.078.3
-  - ldc-1.17.0
-  - ldc-1.16.0
+  - ldc-1.20.1
   - ldc-1.15.0
-  - dmd-2.087.1
-  - dmd-2.086.1
-  - dmd-2.085.1
-  - dmd-2.079.0
-  - dmd-beta
+  - ldc-1.19.0
+  - ldc-1.18.0
+  - dmd-2.089.1
+  - dmd-2.088.1
+  - dmd-nightly
+  - ldc-latest-ci
 
 env:
     - CONFIG=select
@@ -30,16 +30,15 @@ env:
 matrix:
   allow_failures:
     - env: CONFIG=libasync
-    - d: dmd-beta
+    - d: dmd-nightly
+    - d: ldc-latest-ci
   exclude:
       - os: linux
         env: CONFIG=kqueue
       - os: osx
         env: CONFIG=epoll
       # see https://github.com/ldc-developers/ldc/issues/2187
-      - d: ldc-1.17.0
-        os: osx
-      - d: ldc-1.16.0
+      - d: ldc-1.18.0
         os: osx
       - d: ldc-1.15.0
         os: osx