Merge pull request #629 from Anton-Latukha/upd-Travis-add-GHC-8.10

CI: Travis: make GHC 8.10 mandatory, and move SDist check onto it

.travis.yml

@@ -95,14 +95,14 @@ jobs:
         - buildStrictly=true
       os: linux
       dist: bionic
-    - name: GHC 8.8.3, macOS, SDist
+    - name: GHC 8.8.3, macOS
       env:
         - GHCVERSION=ghc883
-        - buildFromSdist=true
       os: osx
-    - name: GHC 8.10.1 Linux
+    - name: GHC 8.10.1, Linux, SDist
       env:
         - GHCVERSION=ghc8101
+        - buildFromSdist=true
       os: linux
       dist: bionic
     - name: GHCJS, Linux
@@ -112,7 +112,6 @@ jobs:
       dist: bionic
   allow_failures:
     - env: GHCVERSION=ghcjs
-    - env: GHCVERSION=ghc8101
   #  2020-05-26: NOTE: Discard macOS GHCJS build + -> CI loop becomes ~30-40 minutes shorter
   exclude:
     - os: osx