Don't install packages needlessly on Travis

.travis.yml

@@ -11,9 +11,31 @@ matrix:
   - stage: "Stage 1"
     env: JOB=autotools ENV=linux
     compiler: gcc
+    addons:
+      apt: &apt-dependencies
+        sources:
+        - avsm
+        - hvr-ghc
+        packages:
+        - alex-3.1.7            # For hstox.
+        - cabal-install-1.18    # For hstox.
+        - check                 # For tests.
+        - ghc-7.8.4             # For hstox.
+        - happy-1.19.5          # For hstox.
+        - libcv-dev             # For av_test.
+        - libhighgui-dev        # For av_test.
+        - libopencv-contrib-dev # For av_test.
+        - libsndfile1-dev       # For av_test.
+        - libvpx-dev            # For toxav.
+        - opam                  # For apidsl and Frama-C.
+        - aspcud                # For Opam
+        - portaudio19-dev       # For av_test.
+        - texinfo               # For libconfig.
   - stage: "Stage 1"
     env: JOB=toxcore ENV=linux
     compiler: clang
+    addons:
+      apt: *apt-dependencies
   - stage: "Stage 1"
     env: JOB=toxcore ENV=windows ARCH=i686
     group: deprecated-2017Q4
@@ -50,27 +72,6 @@ matrix:
     os: osx
   fast_finish: true
 
-addons:
-  apt:
-    sources:
-    - avsm
-    - hvr-ghc
-    packages:
-    - alex-3.1.7            # For hstox.
-    - cabal-install-1.18    # For hstox.
-    - check                 # For tests.
-    - ghc-7.8.4             # For hstox.
-    - happy-1.19.5          # For hstox.
-    - libcv-dev             # For av_test.
-    - libhighgui-dev        # For av_test.
-    - libopencv-contrib-dev # For av_test.
-    - libsndfile1-dev       # For av_test.
-    - libvpx-dev            # For toxav.
-    - opam                  # For apidsl and Frama-C.
-    - aspcud                # For Opam
-    - portaudio19-dev       # For av_test.
-    - texinfo               # For libconfig.
-
 cache:
   directories:
   # Although Travis documentation says not to rely on the value of $HOME, we