try fixing travix build by not using sudo if we don't have it available

.travis.yml

@@ -32,11 +32,6 @@ branches:
   only:
     - master
 
-before_install:
-  - sudo apt-get update
-  - sudo apt-get install gdb python$( python -c 'import sys; print("%d.%d" % sys.version_info[:2])' )-dbg || true
-  - dpkg -l gdb || true
-
 install:
   - CFLAGS="-O2 -ggdb -Wall -Wextra $(python -c 'import sys; print("-fno-strict-aliasing" if sys.version_info[0] == 2 else "")')" python setup.py build
 
@@ -58,3 +53,10 @@ matrix:
       env: BACKEND=cpp
     - python: pypy3
       env: BACKEND=cpp
+
+addons:
+  apt:
+    packages:
+      - gdb
+      - python-dbg
+      - python3-dbg