travis: replace Trusty-based LLVM toolchain apt-get with apt addon

Trusty's LLVM toochain repository was whitelisted some time ago. See:
https://github.com/travis-ci/apt-source-whitelist/commit/479067c5e74cb0c1e2419209179b1afe2edce274

Signed-off-by: Andres Gomez <agomez@igalia.com>
[Emil Velikov]
 - set sudo to false
 - reference the Trusty change (Rhys)
 - keep libedit-dev
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>

.travis.yml

@@ -1,6 +1,6 @@
 language: c
 
-sudo: required
+sudo: false
 dist: trusty
 
 cache:
@@ -9,6 +9,8 @@ cache:
 
 addons:
   apt:
+    sources:
+      - llvm-toolchain-trusty-3.9
     packages:
       - libdrm-dev
       - x11proto-xf86vidmode-dev
@@ -17,6 +19,7 @@ addons:
       - libx11-xcb-dev
       # LLVM packaging is broken and misses these dependencies
       - libedit-dev
+      - llvm-3.9-dev
       - libelf-dev
       - scons
 
@@ -33,7 +36,6 @@ env:
     - LIBXCB_VERSION=libxcb-1.11
     - LIBXSHMFENCE_VERSION=libxshmfence-1.2
     - LLVM_VERSION=3.9
-    - LLVM_PACKAGE="llvm-${LLVM_VERSION} llvm-${LLVM_VERSION}-dev"
     - LLVM_CONFIG="llvm-config-${LLVM_VERSION}"
     - PKG_CONFIG_PATH=$HOME/prefix/lib/pkgconfig
     - LD_LIBRARY_PATH="$HOME/prefix/lib:$LD_LIBRARY_PATH"
@@ -91,15 +93,6 @@ install:
   - tar -jxvf $LIBXSHMFENCE_VERSION.tar.bz2
   - (cd $LIBXSHMFENCE_VERSION && ./configure --prefix=$HOME/prefix && make install)
 
-  # Install LLVM directly via apt-get (not Travis-CI's apt addon)
-  # See https://github.com/travis-ci/apt-source-whitelist/pull/205#issuecomment-216054237
-
-  - wget -nv -O - http://llvm.org/apt/llvm-snapshot.gpg.key | sudo apt-key add -
-  - sudo apt-add-repository -y 'deb http://llvm.org/apt/trusty llvm-toolchain-trusty-3.9 main'
-  - sudo apt-add-repository -y 'deb http://llvm.org/apt/trusty llvm-toolchain-trusty main'
-  - sudo apt-get update -qq
-  - sudo apt-get install -qq -y $LLVM_PACKAGE
-
 script:
   - if test "x$BUILD" = xmake; then
       ./autogen.sh --enable-debug