Remove dub install in before_install

Use the unofficial travis syntax instead.

.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-2.091.0
-  - dmd-2.078.3
-  - ldc-1.20.1
-  - ldc-1.15.0
-  - ldc-1.19.0
-  - ldc-1.18.0
-  - dmd-2.089.1
-  - dmd-2.088.1
-  - dmd-nightly
-  - ldc-latest-ci
+  - dmd-2.091.0,dub
+  - dmd-2.078.3,dub
+  - ldc-1.20.1,dub
+  - ldc-1.15.0,dub
+  - ldc-1.19.0,dub
+  - ldc-1.18.0,dub
+  - dmd-2.089.1,dub
+  - dmd-2.088.1,dub
+  - dmd-nightly,dub
+  - ldc-latest-ci,dub
 
 env:
     - CONFIG=select
@@ -54,11 +54,6 @@ branches:
   only:
     - master
 
-before_install:
-  - wget https://dlang.org/install.sh -O ~/dlang/install.dub.sh
-  - . $(bash ~/dlang/install.dub.sh -a dub)
-  - dub --version
-
 script: ./travis-ci.sh
 
 after_success: