Switch to Docker based builds on Travis

Since Gammu PPA is now whitelisted, we can use Docker infrastructure on
Travis which will bring faster builds.

Signed-off-by: Michal Čihař <michal@cihar.com>

.travis.yml

@@ -7,10 +7,6 @@ python:
   - "3.4"
   - "pypy"
   - "pypy3"
-before_install:
-  - sudo add-apt-repository ppa:nijel/ppa -y
-  - sudo apt-get update
-  - sudo apt-get install libgammu-dev libdbd-sqlite3
 install:
   - pip install -r requirements.txt
 script:
@@ -26,6 +22,9 @@ matrix:
   allow_failures:
     - python: "pypy"
     - python: "pypy3"
+# use docker based faster build
+sudo: false
+# install build dependencies and enable Coverity scan
 addons:
   coverity_scan:
     project:
@@ -34,3 +33,9 @@ addons:
     notification_email: michal@cihar.com
     build_command: ./setup.py build
     branch_pattern: coverity_scan
+  apt:
+    sources:
+    - gammu
+    packages:
+    - libgammu-dev
+    - libdbd-sqlite3