travis: Don't install recommended packages via apt

This passes --no-install-recommends both for Dockerfiles and Travis
itself.

misc/docker/debian-jessie/Dockerfile

@@ -5,9 +5,17 @@ ENV DEBIAN_FRONTEND=noninteractive
 
 RUN apt-get -y update && \
     apt-get -y dist-upgrade && \
-    apt-get -y install python3-pyqt5 python3-pyqt5.qtwebkit python-tox \
-                       python3-sip xvfb git python3-setuptools wget \
-                       herbstluftwm locales
+    apt-get -y install --no-install-recommends \
+            python3-pyqt5 \
+            python3-pyqt5.qtwebkit \
+            python-tox \
+            python3-sip \
+            xvfb \
+            git \
+            python3-setuptools \
+            wget \
+            herbstluftwm \
+            locales
 RUN echo 'en_US.UTF-8 UTF-8' > /etc/locale.gen && locale-gen
 
 RUN useradd user && mkdir /home/user && chown -R user:users /home/user

misc/docker/ubuntu-wily/Dockerfile

@@ -5,9 +5,17 @@ ENV DEBIAN_FRONTEND=noninteractive
 
 RUN apt-get -y update && \
     apt-get -y dist-upgrade && \
-    apt-get -y install python3-pyqt5 python3-pyqt5.qtwebkit python-tox \
-                       python3-sip xvfb git python3-setuptools wget \
-                       herbstluftwm language-pack-en
+    apt-get -y install --no-install-recommends \
+            python3-pyqt5 \
+            python3-pyqt5.qtwebkit \
+            python-tox \
+            python3-sip \
+            xvfb \
+            git \
+            python3-setuptools \
+            wget \
+            herbstluftwm \
+            language-pack-en
 
 RUN useradd user && mkdir /home/user && chown -R user:users /home/user
 USER user

scripts/dev/ci/install.py

@@ -137,7 +137,7 @@ elif TRAVIS_OS == 'linux':
 
     if pkgs:
         apt_get(['update'])
-        apt_get(['install'] + pkgs)
+        apt_get(['install', '--no-install-recommends'] + pkgs)
 
     if TESTENV == 'flake8':
         apt_get(['update'])