.travis.yml: Allow forks to build

But then exclude builds in forks from the notifications.

.travis.yml

@@ -9,18 +9,9 @@ dist: trusty
 sudo: true
 
 # We whitelist branches, as we don't really need to build dev-branches.
+# Except if those branches are in a fork.
 # Remember to add release branches, both here and to appveyor.yml.
-branches:
-  only:
-    - master
-    - "3.0"
-    - "2.4"
-    - "2.2"
-    - "2.0"
-    - "1.24"
-    - "1.22"
-    - "1.20"
-    - "1.18"
+if: branch IN (master, 3.0, 2.4, 2.2, 2.0, 1.24, 1.22, 1.20, 1.18) OR repo != haskell/cabal
 
 # The following enables several GHC versions to be tested; often it's enough to
 # test only against the last release in a major GHC version. Feel free to omit
@@ -177,6 +168,9 @@ after_success:
 
 notifications:
   irc:
+    if: repo = haskell/cabal
     channels:
       - "chat.freenode.net##haskell-cabal"
-  slack: haskell-cabal:sCq6GLfy9N8MJrInosg871n4
+  slack:
+    if: repo = haskell/cabal
+    rooms: haskell-cabal:sCq6GLfy9N8MJrInosg871n4